The Feast of the Annunciation to the Blessed Virgin Mary is celebrated on March 25 each year, nine months before Christmas. 'Annunciation' means 'announcement'. This feast day marks the visit of the angel Gabriel to Mary, telling her that she would be the mother of the Son of God. The feast celebrates God's action in becoming truly human and living in human flesh. It also celebrates Mary’s acceptance of God's action, representing humanity's acceptance of God's gift of salvation in his Son.

The Magnificat (Luke 1:46-55), said regularly at Evening Prayer, is the poem in which Mary responds to the Annunciation and celebrates the power of God.

Annunciation
John Donne

Salvation to all that will is nigh;
That All, which always is all everywhere,
Which cannot sin, and yet all sins must bear,
Which cannot die, yet cannot choose but die,
Lo, faithful virgin, yields Himself to lie
In prison, in thy womb; and though He there
Can take no sin, nor thou give, yet He will wear,
Taken from thence, flesh, which death's force may try.
Ere by the spheres time was created, thou
Wast in His mind, who is thy Son and Brother;
Whom thou conceivst, conceived; yea thou art now
Thy Maker's maker, and thy Father's mother;
Thou hast light in dark, and shutst in little room,
Immensity cloistered in thy dear womb.

The Song of Mary or Magnificat

My soul proclaims the greatness of the Lord:
     my spirit rejoices in God my Saviour,
Who has looked with favour on his lowly servant:
     From this day all generations will call me blessed;
The Almighty has done great things for me:
     and holy is his name.
God has mercy on those who him:
     from generation to generation.
He has shown strength with his arm:
     and scattered the proud in their conceit,
Casting down the mighty from their thrones:
     and lifting up the lowly.
God has filled the hungry with good things:
     and sent the rich away empty.
He has come to the aid of his servant Israel:
     to remember the promise of mercy,
The promise made to our forebears:
     to Abraham and his children for ever. (Luke 1:47-55)

There are many musical settings of the Magnificat. Here is one of the very finest. Magnificat (BWV 243) by Johann Sebastian Bach, conducted by Nikolaus Harnoncourt.

And Sabine Baring-Gould's masterful Annunciation hymn, itself the translation of a Basque song:

The angel Gabriel from heaven came,
His wings as drifted snow, his eyes as flame;
"All hail," said he, "thou lowly maiden Mary,
Most highly favored lady," Gloria!

"For know a blessed Mother thou shalt be,
All generations laud and honor thee,
Thy Son shall be Emmanuel, by seers foretold,
Most highly favored lady," Gloria!

Then gentle Mary meekly bowed her head,
"To me be as it pleaseth God," she said,
"My soul shall laud and magnify his holy Name."
Most highly favored lady, Gloria!

Of her, Emmanuel, the Christ, was born
in Bethlehem, all on a Christmas morn,
and Christian folk throughout the world will ever say—
"Most highly favored lady," Gloria!