As a tradition, it's as old as civilisation itself. People have gathered together for mid-winter rituals since the beginning of human society. It's all about light and darkness, life and hope. When Christianity came on to the scene it simply superimposed a new meaning onto an ancient tradition.

Others have answered so I will merely add that my brother is a Druid and does not, of course, celebrate 'Christmas'. However, the Druids celebrated midwinter long before Christ appeared in history.... so he celebrates! But not 'Christmas.' As Suki says, it's a new name for a very old tradition. Some customs have been lost, others have arrived from other countries. It's a real mixture. As I am not a Christian I don't really know much about the Christian side of Christmas. All I do is eat a lot of chocolate.

The answer, if you are religious, is that it is the celebration of the birth of Jesus Christ; who was the son of God, born approximately 2000 years ago. A more practical answer: it is part of the traditional western end of year celebration, where family comes together for gift giving and to share a meal.

Linus tells Charlie Brown "what Christmas is all about." http://www.youtube.com/watch?v=kVGJgJ5cReg This is the ending of "A Charlie Brown Christmas." Linus is reading from the Gospel of Luke, written about CE 90. Christmas proper celebrates the nativity or birth of Jesus of Nazareth, who Christians believe is the Word of God made flesh (human). The Christian church recognizes four written Gospels, written proclamations of the 'good news.' 'Good news' is the English translation of the Greek word 'evangelion.' These four written Gospels are Matthew, Mark, Luke and John, named after their purported authors. Note that the Gospels are not biographies, but Christian publications that proclaim the life of Jesus from a Christian perspective. In other words, the Gospels are unabashedly Christian. Anyway, in the youtube segment, Linus quotes the Gospel of Luke, chapter 2, verses 8-14, from the version of the holy bible called the King James Version. The passage is 8 And there were in the same country shepherds abiding in the field, keeping watch over their flock by night. 9 And, lo, the angel of the Lord came upon them, and the glory of the Lord shone round about them: and they were sore afraid. 10 And the angel said unto them, Fear not: for, behold, I bring you good tidings of great joy, which shall be to all people. 11 For unto you is born this day in the city of David a Saviour, which is Christ the Lord. 12 And this shall be a sign unto you; Ye shall find the babe wrapped in swaddling clothes, lying in a manger. 13 And suddenly there was with the angel a multitude of the heavenly host praising God, and saying, 14 Glory to God in the highest, and on earth peace, good will toward men. You can read the entire chapter 2 of the Gospel of Luke here: https://www.biblegateway.com/passage/?search=Luke+2&version=KJV

It IS a story? It's ? You mean - its its / his / her etc IT'S an easy mistake to make.