Although most Christians celebrate December 25 as the birthday of Jesus Christ, few in the first two Christian centuries claimed any knowledge of the exact day or year in which he was born. The oldest existing record of a Christmas celebration is found in a Roman almanac that tells of a Christ's Nativity festival led by the church of Rome in 336 A.D. The precise reason why Christmas came to be celebrated on December 25 remains obscure, but most researchers believe that Christmas originated as a Christian substitute for pagan celebrations of the winter solstice.
To early Christians (and to many Christians today), the most important holiday on the Christian calendar was Easter, which commemorates the death and resurrection of Jesus Christ. However, as Christianity began to take hold in the Roman world, in the early fourth century, church leaders had to contend with a popular Roman pagan holiday commemorating the "birthday of the unconquered sun" (natalis solis invicti)--the Roman name for the winter solstice.
Every winter, Romans honored the pagan god Saturn, the god of agriculture, with a festival that began on December 17 and usually ended on or around December 25 with a winter-solstice celebration in honor of the beginning of the new solar cycle. This festival was a time of merrymaking, and families and friends would exchange gifts. At the same time, Mithraism--worship of the ancient Persian god of light--was popular in the Roman army, and the cult held some of its most important rituals on the winter solstice.
After the Roman Emperor Constantine I converted to Christianity in 312 and sanctioned Christianity, church leaders made efforts to appropriate the winter-solstice holidays and thereby achieve a more seamless conversion to Christianity for the emperor's subjects. In rationalizing the celebration of Jesus' birthday in late December, church leaders may have argued that since the world was allegedly created on the spring equinox (late March), so too would Jesus have been conceived by God on that date. The Virgin Mary, pregnant with the son of God, would hence have given birth to Jesus nine months later on the winter solstice.
From Rome, the Christ's Nativity celebration spread to other Christian churches to the west and east, and soon most Christians were celebrating Christ's birth on December 25. To the Roman celebration was later added other winter-solstice rituals observed by various pagan groups, such as the lighting of the Yule log and decorations with evergreens by Germanic tribes. The word Christmas entered the English language originally as Christes maesse, meaning "Christ's mass" or "festival of Christ" in Old English. A popular medieval feast was that of St. Nicholas of Myra, a saint said to visit children with gifts and admonitions just before Christmas. This story evolved into the modern practice of leaving gifts for children said to be brought by "Santa Claus," a derivative of the Dutch name for St. Nicholas--Sinterklaas.