The commemoration of Christ's nativity. The earliest mention of its being celebrated on 25 Dec. is in the Philocalian Calendar, which represents Roman practice in 336. The date was probably chosen to oppose the pagan feast of the Natalis Solis Invicti by a celebration of the birth of the ‘Sun of Righteousness’. Another tradition derived the date of Christmas from that of the Annunciation, held to have taken place on the same day of the year as the Crucifixion and calculated as 25 Mar. After the accession of Constantine, the observance of 25 Dec. in the W. spread from Rome. In the E. the closely related feast of the Epiphany (6 Jan.) was at first more important; by the later 4th cent. this commemorated the Nativity as well as the Baptism of Christ. By the mid-5th cent. most of the E. had adopted 25 Dec.
The day is celebrated in the W. rite by three Masses, of the night (normally at midnight), of the dawn, and of the day.