Christmas is the commemoration of the calling and anointing of Jesus Christ as the Savior of the world.
The first Christmas celebration happened in a pre-mortal state when, according to scripture, "... the morning stars sang together and the sons of God shouted for joy." (Job 38:7)
This celebration took place "before the world was" in a gathering of the spirit children of God the Father.
All of the souls of humankind who have lived or who will live on the face of the earth were present at this event.
God's plan for man on this earth included gaining a physical body and experiencing the agency of making choices of conscience. His intent was to provide the means for each spirit to grow in intelligence and to gain the experiences necessary to become more like God himself.
The physical existence of humankind on this earth means a temporary separation from God's presence. Since no unclean thing can bear God's presence in the hereafter, a way needed to be provided for His spirit children to be redeemed from the inevitable sin that could forever separate the eternal family. His plan provided a way.
Jesus, as the first born of the Father, possesses the God-like attributes necessary of a Savior. The word "Christ" is actually a title, not a name. It means "Messiah" or "anointed one".
Jesus, near the end of his mortal mission, referred to his pre-existent condition: "And this is life eternal, that they know thee the only true God, and Jesus Christ, whom thou hast sent. I have glorified thee on the earth: I have finished the work thou gavest me to do. And now, O Father, glorify me with thine ownself with the glory I had with thee before the world was." (John 17:3-5)
The anointing of Jesus Christ to this calling was a great gift given freely to all the spirits of men. "For God so loved the world that he gave his only begotten Son, that whosoever believeth on him shall not perish, but have everlasting life." (John 3:16) Christmas celebrates the giving of this gift and our acceptance of it in the pre-mortal realm.
Christmas is a timeless celebration. This event was known and celebrated by the ancients in the centuries before the birth of Jesus Christ. Adam, Enoch, Noah, Abraham, Moses, Isaiah, Micah and other Old Testament prophets make reference to the pre-earth events identifying Jesus as the Christ.
The Nativity, or Birth of Jesus Christ, is the hallmark event of Christmas. Many mistakenly call this mortal entrance of Jesus the first Christmas. Similar to the great host of spirits who "shouted for joy" in the pre-existent state, the birth of Jesus was heralded by a "heavenly host" of angels witnessed by the shepherds.
Many societies celebrate Christmas today. There are many methods and elements of Christmas celebration. Through the passage of time some of the deeper spiritual meaning originally associated with Christmas has been forgotten or displaced as Christmas has become more of a secular event rather than a religious observance.