The Annunciation (from Latin annuntiatio), also referred to as the Annunciation to the Blessed Virgin Mary, the Annunciation of Our Lady, or the Annunciation of the Lord, is the Christian celebration of the announcement by the angel Gabriel to Mary that she would conceive and bear a son through a virgin birth and become the mother of Jesus Christ, the Christian Messiah and Son of God, marking the Incarnation. Gabriel told Mary to name her son Jesus, meaning "YHWH is salvation".

According to Luke 1:26, the Annunciation occurred "in the sixth month" of Elizabeth's pregnancy with John the Baptist. Many Christians observe this event with the Feast of the Annunciation on 25 March, an approximation of the northern vernal equinox nine full months before Christmas, the ceremonial birthday of Jesus. The Annunciation is a key topic in Christian art in general, as well as in Marian art in the Catholic Church, having been especially prominent during the Middle Ages and Renaissance. A work of art depicting the Annunciation is sometimes itself called an Annunciation.

The feast of the Annunciation is usually held on 25 March. It is moved in the Catholic Church, Anglican and Lutheran liturgical calendars when that date falls during Holy Week or Easter Week or on a Sunday. The Eastern Orthodox Church, Oriental Orthodoxy, and Eastern Catholic Churches do not move the feast. Instead they have special combined liturgies for those years when the Annunciation coincides with another feast. In these churches, even on Good Friday a Divine Liturgy is celebrated when it coincides with the Annunciation.

Greek Independence Day is celebrated on the feast of the Annunciation and 25 March is also a national holiday in the Lebanon.

When the calendar system of Anno Domini was first introduced by Dionysius Exiguus in AD 525, he assigned the beginning of the new year to 25 March since, according to Catholic theology, the era of grace began with the Incarnation of Christ. The first certain mentions of the feast are in a canon of the 656 Council of Toledo, where it is described as celebrated throughout the church. The 692 Council of Constantinople "in Trullo" forbade observance of any festivals during Lent, excepting Sunday and the Feast of the Annunciation. An earlier origin had been claimed for it on the grounds that it appeared in manuscripts of the sermons of Athanasius and Gregory Thaumaturgus but they were subsequently discovered to be spurious.

Along with Easter, 25 March was used as the New Year's Day in many pre-modern Christian countries. The holiday was moved to January 1 in France by Charles IX's 1564 Edict of Roussillon. In England, the feast of the Annunciation came to be known as Lady Day, and Lady Day marked the beginning of the English new year until 1752. Also in England, the 1240 Synod of Worcester banned all servile work during the Feast of the Annunciation, making it a day of rest.