December 25th (on the Gregorian calendar) is the traditional day celebrating the [re]birth of the Sun in many pre-Christian Pagan cultures. Early Christian church leaders are presumed to have used the date for the celebration of the birth of Jesus Christ (the Son of God) so as to help win over converts to the dawning religion.
It is also a date on which many contemporary Christians celebrate the incarnation. As part of their religious faith they believe that God became man and visited the Earth to make redemption available to all who would so believe.