Though the Gospels of Mathew and Luke both give an account of Christ’s birth, neither one provides a date for this great event. In regard to the chronology of Christ’s life, Early Church leaders were primarily concerned with determining the date of Christ death in order to determine the celebration of his resurrection (Easter).
The Catholic Church, from at least the second century, fixed December 25 as the date Jesus Christ was born. This date was chosen in order to replace the pagan Roman festival of Saturnalia. Saturnalia was a popular winter festival and so the Catholic Church wisely substituted Christmas in its place.
The church could have chosen another date on which to celebrate the birth of Christ. Another reason why December 25 may have been deemed suitable is its proximity to the winter solstice. After that date the days start to become longer, and thus it is the beginning of a season of light entering the world (cf. John 1:5).the summer solstice- after which the days start to get shorter- falls near June24, on which the church celebrates the birth of John the Baptist, who declared of Christ,” He must increase, but I must decrease” (john 3:30).