Christmas celebrates the birth of Jesus Christ in Bethlehem — the central mystery of the Incarnation: that the eternal Word through whom all things were made took flesh in the womb of the Virgin Mary and was born as a human infant. Observed on December 25 since at least the early fourth century in Rome, it has become the most universally celebrated Christian feast, though its theological weight lies not in sentiment but in the staggering claim that 'the Word became flesh and dwelt among us' (John 1:14).

The midnight birth tradition — that Christ was born at the darkest hour of the longest night — is ancient though not scriptural. The ox and ass at the manger derive not from the Gospels but from Isaiah 1:3 ('The ox knows its owner, and the donkey its master's crib') and appear in Christian art from at least the fourth century. The cave tradition (rather than a wooden stable) comes from Justin Martyr (Dialogue with Trypho 78, c. 160) and the Protoevangelium of James, and is reflected in the Church of the Nativity in Bethlehem, built over a cave. Archaeological evidence supports continuous veneration at this site from at least the fourth century.

The Nativity is narrated in two complementary Gospel accounts. Luke (2:1-20) places the birth in Bethlehem during a census ordered by Augustus, when Joseph and Mary traveled from Nazareth. Finding no room in the lodging place, Mary gave birth and laid the infant in a manger. Shepherds watching their flocks by night received the angelic announcement — 'To you is born this day in the city of David a Savior, who is Christ the Lord' — and came to see the child. Matthew (1:18-2:12) focuses on Joseph's dream, the visit of the Magi, and the flight to Egypt.

The December 25 date appears first in the Roman Chronograph of 354, reflecting practice from approximately 336 CE. The Calculation Hypothesis — that early Christians believed Jesus was conceived and died on the same date (March 25, the spring equinox), placing birth nine months later — is attested in Hippolytus and Sextus Julius Africanus (c. 221). The Sol Invictus theory, once popular, is now questioned by recent scholarship (Hijmans) since the pagan festival may not have been fixed to December 25 before the Christian celebration.