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Christmas (Old English: Crīstesmæsse, meaning "Christ's Mass") is an annual commemoration of the birth of Jesus Christ and a widely observed holiday, celebrated generally on December 25 by millions of people around the world.

A feast central to the Christian liturgical year, it closes the Advent season and initiates the twelve days of Christmastide.Christmas is a civil holiday in many of the world's nations,is celebrated by an increasing number of non-Christians,and is an integral part of the Christmas and holiday season.

The precise year of Jesus' birth, which some historians place between 7 and 2 BC, is unknown. His birth is mentioned in two of the four canonical gospels. By the early-to-mid 4th century, the Western Christian Church had placed Christmas on December 25, a date later adopted in the East.

The date of Christmas may have initially been chosen to correspond with the day exactly nine months after early Christians believed Jesus to have been conceived, as well as the date of celebration of the southern solstice (i.e., the Roman winter solstice), with a sun connection being possible because Christians consider Jesus to be the "Sun of righteousness" prophesied in Malachi 4:2.

The original date of the celebration in Eastern Christianity was January 6, in connection with Epiphany, and that is still the date of the celebration for the Armenian Apostolic Church and in Armenia, where it is a public holiday. As of 2012, there is a difference of 13 days between the modern Gregorian calendar and the older Julian calendar.

Those who continue to use the Julian calendar or its equivalents thus celebrate December 25 and January 6 on what for the majority of the world is January 7 and January 19. For this reason, Ethiopia, Russia, Ukraine, Serbia, the Republic of Macedonia, and the Republic of Moldova celebrate Christmas on what in the Gregorian calendar is January 7; the Church of Greece and all Greek Orthodox Churches celebrate Christmas on December 25.

The popular celebratory customs associated in various countries with Christmas have a mix of pre-Christian, Christian, and secular themes and origins.

Popular modern customs of the holiday include gift giving, Christmas music and caroling, an exchange of Christmas cards, church celebrations, a special meal, and the display of various Christmas decorations, including Christmas trees, Christmas lights, nativity scenes, garlands, wreaths, mistletoe, and holly.

In addition, several closely related and often interchangeable figures, known as Santa Claus, Father Christmas, Saint Nicholas, and Christkind, are associated with bringing gifts to children during the Christmas season and have their own body of traditions and lore.

Because gift-giving and many other aspects of the Christmas festival involve heightened economic activity among both Christians and non-Christians, the holiday has become a significant event and a key sales period for retailers and businesses. The economic impact of Christmas is a factor that has grown steadily over the past few centuries in many regions of the world.

Commemorating Jesus' birth
Christians celebrate the birth of Jesus to the Virgin Mary as a fulfillment of the Old Testament's Messianic prophecy. The Bible contains two accounts which describe the events surrounding Jesus' birth. Depending on one's perspective, these accounts either differ from each other or tell two versions of the same story.

These biblical accounts are found in the Gospel of Matthew, namely Matthew 1:18, and the Gospel of Luke, specifically Luke 1:26 and 2:40. According to these accounts, Jesus was born to Mary, assisted by her husband Joseph, in the city of Bethlehem.

On Christmas Day, the Christ Candle in the center of the Advent wreath is traditionally lit in many church services.

According to popular tradition, the birth took place in a stable, surrounded by farm animals. A manger (that is, a feeding trough) is mentioned in Luke 2:7, where it states Mary "wrapped him in swaddling clothes and laid him in a manger, because there was no room for them in the inn" (KJV); and "She wrapped him in cloths and placed him in a manger, because there was no guest room available for them" (NIV). Shepherds from the fields surrounding Bethlehem were told of the birth by an angel, and were the first to see the child.

Popular tradition also holds that three kings or wise men (named Melchior, Caspar, and Balthazar) visited the infant Jesus in the manger, though this does not strictly follow the Biblical account.

The Gospel of Matthew instead describes a visit by an unspecified number of magi, or astrologers, sometime after Jesus was born while the family was living in a house (Matthew 2:11), who brought gifts of gold, frankincense, and myrrh to the young child Jesus.

The visitors were said to be following a mysterious star, commonly known as the Star of Bethlehem, believing it to announce the birth of a king of the Jews. The commemoration of this visit, the Feast of Epiphany celebrated on January 6, is the formal end of the Christmas season in some churches.

Christians celebrate Christmas in various ways. In addition to this day being one of the most important and popular for the attendance of church services, there are other devotions and popular traditions. In some Christian denominations, children re-enact the events of the Nativity with animals to portray the event with more realism or sing carols that reference the event.

Some Christians also display a small re-creation of the Nativity, known as a Nativity scene or crèche, in their homes, using figurines to portray the key characters of the event. Prior to Christmas Day, the Eastern Orthodox Church practices the 40-day Nativity Fast in anticipation of the birth of Jesus, while much of Western Christianity celebrates four weeks of Advent.

The final preparations for Christmas are made on Christmas Eve, and many families' major observation of Christmas actually falls in the evening of this day.

A long artistic tradition has grown of producing painted depictions of the nativity in art. Nativity scenes are traditionally set in a stable with livestock and include Mary, Joseph, the infant Jesus in the manger, the three wise men, the shepherds and their sheep, the angels, and the Star of Bethlehem.

Source: Wikipedia

A computer is a general purpose device that can be programmed to carry out a finite set of arithmetic or logical operations. Since a sequence of operations can be readily changed, the computer can solve more than one kind of problem.

Conventionally, a computer consists of at least one processing element, typically a central processing unit (CPU) and some form of memory. The processing element carries out arithmetic and logic operations, and a sequencing and control unit that can change the order of operations based on stored information. Peripheral devices allow information to be retrieved from an external source, and the result of operations saved and retrieved.

The first electronic digital computers were developed between 1940 and 1945 in the United Kingdom and United States. Originally they were the size of a large room, consuming as much power as several hundred modern personal computers (PCs). In this era mechanical analog computers were used for military applications.

Modern computers based on integrated circuits are millions to billions of times more capable than the early machines, and occupy a fraction of the space. Simple computers are small enough to fit into mobile devices, and mobile computers can be powered by small batteries. Personal computers in their various forms are icons of the Information Age and are what most people think of as "computers". However, the embedded computers found in many devices from mp3 players to fighter aircraft and from toys to industrial robots are the most numerous.

History of computing
The first use of the word "computer" was recorded in 1613, referring to a person who carried out calculations, or computations, and the word continued with the same meaning until the middle of the 20th century. From the end of the 19th century the word began to take on its more familiar meaning, a machine that carries out computations.

Limited-function early computersThe history of the modern computer begins with two separate technologies, automated calculation and programmability, but no single device can be identified as the earliest computer, partly because of the inconsistent application of that term.

A few devices are worth mentioning though, like some mechanical aids to computing, which were very successful and survived for centuries until the advent of the electronic calculator, like the Sumerian abacus, designed around 2500 BC of which a descendant won a speed competition against a modern desk calculating machine in Japan in 1946, the slide rules, invented in the 1620s, which were carried on five Apollo space missions, including to the moon and arguably the astrolabe and the Antikythera mechanism, an ancient astronomical computer built by the Greeks around 80 BC.

The Greek mathematician Hero of Alexandria (c. 10–70 AD) built a mechanical theater which performed a play lasting 10 minutes and was operated by a complex system of ropes and drums that might be considered to be a means of deciding which parts of the mechanism performed which actions and when.This is the essence of programmability.

Around the end of the 10th century, the French monk Gerbert d'Aurillac brought back from Spain the drawings of a machine invented by the Moors that answered either Yes or No to the questions it was asked.Again in the 13th century, the monks Albertus Magnus and Roger Bacon built talking androids without any further development (Albertus Magnus complained that he had wasted forty years of his life when Thomas Aquinas, terrified by his machine, destroyed it).

In 1642, the Renaissance saw the invention of the mechanical calculator, a device that could perform all four arithmetic operations without relying on human intelligence. The mechanical calculator was at the root of the development of computers in two separate ways. Initially, it was in trying to develop more powerful and more flexible calculators that the computer was first theorized by Charles Babbage and then developed. Secondly, development of a low-cost electronic calculator, successor to the mechanical calculator, resulted in the development by Intel of the first commercially available microprocessor integrated circuit.

First general-purpose computers
In 1801, Joseph Marie Jacquard made an improvement to the textile loom by introducing a series of punched paper cards as a template which allowed his loom to weave intricate patterns automatically. The resulting Jacquard loom was an important step in the development of computers because the use of punched cards to define woven patterns can be viewed as an early, albeit limited, form of programmability.

It was the fusion of automatic calculation with programmability that produced the first recognizable computers. In 1837, Charles Babbage was the first to conceptualize and design a fully programmable mechanical computer, his analytical engine.

Limited finances and Babbage's inability to resist tinkering with the design meant that the device was never completed—nevertheless his son, Henry Babbage, completed a simplified version of the analytical engine's computing unit (the mill) in 1888. He gave a successful demonstration of its use in computing tables in 1906. This machine was given to the Science museum in South Kensington in 1910.

In the late 1880s, Herman Hollerith invented the recording of data on a machine-readable medium. Earlier uses of machine-readable media had been for control, not data. "After some initial trials with paper tape, he settled on punched cards ..." To process these punched cards he invented the tabulator, and the keypunch machines.

These three inventions were the foundation of the modern information processing industry. Large-scale automated data processing of punched cards was performed for the 1890 United States Census by Hollerith's company, which later became the core of IBM. By the end of the 19th century a number of ideas and technologies, that would later prove useful in the realization of practical computers, had begun to appear: Boolean algebra, the vacuum tube (thermionic valve), punched cards and tape, and the teleprinter.

During the first half of the 20th century, many scientific computing needs were met by increasingly sophisticated analog computers, which used a direct mechanical or electrical model of the problem as a basis for computation. However, these were not programmable and generally lacked the versatility and accuracy of modern digital computers.

Alan Turing is widely regarded as the father of modern computer science. In 1936 Turing provided an influential formalisation of the concept of the algorithm and computation with the Turing machine, providing a blueprint for the electronic digital computer. Of his role in the creation of the modern computer, Time magazine in naming Turing one of the 100 most influential people of the 20th century, states: "The fact remains that everyone who taps at a keyboard, opening a spreadsheet or a word-processing program, is working on an incarnation of a Turing machine".

The Atanasoff–Berry Computer (ABC) was the world's first electronic digital computer, albeit not programmable. Atanasoff is considered to be one of the fathers of the computer.Conceived in 1937 by Iowa State College physics professor John Atanasoff, and built with the assistance of graduate student Clifford Berry, the machine was not programmable, being designed only to solve systems of linear equations. The computer did employ parallel computation. A 1973 court ruling in a patent dispute found that the patent for the 1946 ENIAC computer derived from the Atanasoff–Berry Computer.

The first program-controlled computer was invented by Konrad Zuse, who built the Z3, an electromechanical computing machine, in 1941.The first programmable electronic computer was the Colossus, built in 1943 by Tommy Flowers.

George Stibitz is internationally recognized as a father of the modern digital computer. While working at Bell Labs in November 1937, Stibitz invented and built a relay-based calculator he dubbed the "Model K" (for "kitchen table", on which he had assembled it), which was the first to use binary circuits to perform an arithmetic operation.

Later models added greater sophistication including complex arithmetic and programmability. A succession of steadily more powerful and flexible computing devices were constructed in the 1930s and 1940s, gradually adding the key features that are seen in modern computers. The use of digital electronics (largely invented by Claude Shannon in 1937) and more flexible programmability were vitally important steps, but defining one point along this road as "the first digital electronic computer" is difficult.Shannon 1940 Notable achievements include:

Konrad Zuse's electromechanical "Z machines". The Z3 (1941) was the first working machine featuring binary arithmetic, including floating point arithmetic and a measure of programmability. In 1998 the Z3 was proved to be Turing complete, therefore being the world's first operational computer.

The non-programmable Atanasoff–Berry Computer (commenced in 1937, completed in 1941) which used vacuum tube based computation, binary numbers, and regenerative capacitor memory. The use of regenerative memory allowed it to be much more compact than its peers (being approximately the size of a large desk or workbench), since intermediate results could be stored and then fed back into the same set of computation elements.

The secret British Colossus computers (1943), which had limited programmability but demonstrated that a device using thousands of tubes could be reasonably reliable and electronically reprogrammable. It was used for breaking German wartime codes. The Harvard Mark I (1944), a large-scale electromechanical computer with limited programmability.

The U.S. Army's Ballistic Research Laboratory ENIAC (1946), which used decimal arithmetic and is sometimes called the first general purpose electronic computer (since Konrad Zuse's Z3 of 1941 used electromagnets instead of electronics). Initially, however, ENIAC had an inflexible architecture which essentially required rewiring to change its programming.

Source: Wikipedia