The Feast of the Annunciation (falling as it does 9 months before Christmas Day) marks the Angel Gabriel appearing to Mary and telling her that she will bear the son of God. It is also known as "Lady Day". The first words of the Angel's salutation, "Hail Mary, full of grace, the Lord is with thee" form the opening of the prayer known as the Hail Mary.

It is celebrated in all branches of Christianity, though the Marian element means it tends to be more prominent in Catholicism and Orthodoxy rather than Protestant denominations.

Its proximity to the Spring Equinox is linked to the belief that creation itself began on that day, as did Christ's redemption of it. But as with many Christian festivals, it is probably fair to say that the early church showed a degree of pragmatism about borrowing the dates of earlier Pagan rituals!

If the 25th of March falls during Holy Week, the festival is moved for the year in question.

More Info: en.wikipedia.org