March 25 marks the Solemnity of the Annunciation. This is the feast day celebrating when the Archangel Gabriel came to the Blessed Virgin Mary and "announced" to her that the Lord God had found favor with her, pronouncing her "Full of Grace." It is from this passage in Scripture that we get part of the "Hail Mary:"
Hail Mary, Full of Grace!
The Lord is with thee!
Blessed are you among women!
And blessed is the fruit of thy womb, Jesus!
And the angel being come in, said unto her: Hail, full of grace, the Lord is with thee: blessed art thou among women.
Continuing with Luke 1 comes the "announcement" not only of the favor the Lord has found with her, but that she would conceive and bear a Son:
 And the angel said to her: Fear not, Mary, for thou hast found grace with God.  Behold thou shalt conceive in thy womb, and shalt bring forth a son; and thou shalt call his name Jesus.  He shall be great, and shall be called the Son of the most High; and the Lord God shall give unto him the throne of David his father; and he shall reign in the house of Jacob for ever. And of his kingdom there shall be no end.  And Mary said to the angel: How shall this be done, because I know not man?  And the angel answering, said to her: The Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God.
Be It Done To Me According To Thy Word
After hearing this, the Blessed Virgin gives her fiat, indicating it was not merely "done" to her, but with her full consent and will:
And Mary said: Behold the handmaid of the Lord; be it done to me according to thy word. And the angel departed from her.
Holy Mary, Mother of God!
 And it came to pass, that when Elizabeth heard the salutation of Mary, the infant leaped in her womb. And Elizabeth was filled with the Holy Ghost:  And she cried out with a loud voice, and said: Blessed art thou among women, and blessed is the fruit of thy womb.  And whence is this to me, that the mother of my Lord should come to me?  For behold as soon as the voice of thy salutation sounded in my ears, the infant in my womb leaped for joy.  And blessed art thou that hast believed, because those things shall be accomplished that were spoken to thee by the Lord.
 And Mary said: My soul doth magnify the Lord.  And my spirit hath rejoiced in God my Saviour.  Because he hath regarded the humility of his handmaid; for behold from henceforth all generations shall call me blessed.  Because he that is mighty, hath done great things to me; and holy is his name.  And his mercy is from generation unto generations, to them that fear him.
The Date of Christmas
Long ago it was thought that a person died on the anniversary of their conception. In 33ad Good Friday fell on what we now call March 25th (the Gregorian Calendar had not yet been developed, but we can go back from when it was to figure out the dates). So, contrary to many who believe the date of Christmas was chosen to line up with pagan holidays and/or the Winter Solstice (which since that is December 21st, it doesn't match up with!), in reality the date of Christmas is based upon the date of Good Friday in 33ad! December 25th is nine months after March 25th!