Christmas occasionally referred to as Christmas Day or Christmastide, is an annual Christian holiday celebrated on December 25 or January 7 that marks and honors the birth of Jesus of Nazareth. The nativity of Jesus, which is the basis for the anno Domini system of dating, is thought to have occurred between 7 and 2 BC. December 25 is not thought to be Jesus’ actual date of birth, and the date may have been chosen to correspond with either a Roman festival, or with the winter solstice.

Modern customs of the holiday include gift-giving, church celebrations, and the display of various decorations—including the Christmas tree, lights, mistletoe, nativity scenes and holly. Santa Claus (also referred to as Father Christmas, although the two figures have different origins) is a popular mythological figure often associated with bringing gifts at Christmas.

Christmas is celebrated throughout the Christian population, but is also celebrated by many non-Christians as a secular, cultural festival. Because gift-giving and several other aspects of the holiday involve heightened economic activity among both Christians and non-Christians, Christmas has become a major event for many retailers.

The above description of Christmas comes from Wiki with English language http://en.wikipedia.org/wiki/Christmas. Also there is the Chinese page you can refer with http://zh.wikipedia.org/wiki/%E5%9C%A3%E8%AF%9E%E8%8A%82.