First, since the basis of Christmas is the birth of Jesus, the Son of God, a distinctly Christian event, even the secular celebration has retained much of the Christian trappings, and it becomes just a part of the celebration.
Second, during this time of year people, realizing that Christmas, at least the event that it celebrates, is inextricably tied to the birth of the Son of God, are brought to at least a superficial realization of their neglect of the things of God, and to sooth their conscience they will attend a church service or two this time of year. In church we sometimes refer to these people Twicers, because the same thing happens on Easter, and Easter and Christmas become the two days each year when some will venture into a church.