The Annunciation celebrates the angel Gabriel's announcement to the Virgin Mary that she would conceive and bear the Son of God (Luke 1:26-38) — the moment when, in the words of the Nicene Creed, the Son of God 'was incarnate by the Holy Spirit of the Virgin Mary.' It is simultaneously a Christological feast (celebrating the Incarnation) and a Marian feast (celebrating Mary's faithful 'yes' — her fiat). Observed on March 25, exactly nine months before Christmas, the feast is the theological hinge of the Christian year: the Incarnation that makes the Nativity, Passion, and Resurrection possible.

In England, March 25 was known as Lady Day and served as the legal New Year from 1155 to 1752, when the Gregorian calendar moved the new year to January 1. Lady Day was the first of four English Quarter Days (with Midsummer, Michaelmas, and Christmas), marking when agricultural contracts began and rents were due. The theological symbolism was intentional: the year began where the story of salvation began, with the Word becoming flesh.

Irenaeus of Lyons (c. 180) developed the foundational New Eve theology: 'The knot of Eve's disobedience was untied by Mary's obedience.' Just as death entered through a woman's 'no' to God's design, salvation entered through a woman's 'yes.' This patristic theme — Mary as the faithful counterpart to faithless Eve — became central to Christian reflection on the Annunciation across all traditions.

The earliest evidence for a March 25 Annunciation feast comes from sixth-century Constantinople (c. 555, under Justinian I). In the Latin West, the Sacramentary of Pope Gelasius (d. 496) and the Sacramentary of Pope Gregory (d. 604) mention it, though scholars debate whether the feast originated in their time or later. The first certain reference is the 656 Council of Toledo, which describes the feast as 'celebrated throughout the church.' The Spanish Church initially observed the Annunciation on December 18 rather than March 25, and the tenth Synod of Toledo (656) confirmed this local usage.

The Quinisext Council (Constantinople, 692) allowed the Annunciation as an exception to the rule against feast days during Lent — a concession reflecting the feast's importance and the practical tension between its fixed March 25 date and the moveable Lenten calendar. Modern Western practice transfers the feast when it falls in Holy Week or Easter Week.