If there's one word that brings common ground to the varied forms in which Christmas is celebrated, it's tradition. Christmas traditions can be as secular as a recipe for ginger cookies or as deep and sacred as a retelling of the nativity of Jesus as recorded in the New Testament. The four Gospels—Matthew, Mark, Luke, and John—are primary sources for understanding the meaning of Christmas.
Tradition, as Bible scholars use the word, is far from trivial. Tradition refers to a theme and supportive facts or details, passed down from generation to generation, for which there can be various perspectives. The modern Christmas story, complete with the shepherds and wise men bringing gifts to the baby Jesus, is actually based on a combination of traditional themes from just two of the Gospels. While all four Gospels describe the crucifixion and resurrection, only two of them include accounts of Jesus' birth.
The Gospel of Mark, probably the first to have been written, opens with Jesus at the age of 30, presenting himself to John the Baptist and subsequently beginning his public ministry. John's Gospel, usually considered the last one written, likewise is silent about the birth, connecting Jesus with the "Word" from "the beginning." See John 1:1. Matthew's and Luke's Gospels were both composed long after Jesus' resurrection and begin with his birth. But the two accounts have significant differences, perhaps mirroring their intended audiences.