The Annunciation of Mary, 25th March

The Annunciation also referred to as the Annunciation to the Blessed Virgin Mary, the Annunciation of Our Lady or Annunciation of the Lady

The Annunciation of Mary is when the Archangel Gabriel visits Mary to say that she is going to have a son, who she was to call Jesus. *
(See Luke ch. 1 verses 26-38)
Mary is surprised because she asks, How can this be I do nor know a man?
She is still a virgin who has only recently become betrothed to Joseph.

The angel replies with these words
The Holy Spirit will come upon you, and the power of the Highest will overshadow you; therefore also, that Holy One who is to be born will be called the Son of God.
Gabriel goes on to explain that her cousin Elizabeth, in old age, is going to give birth to a son. (This son is John the Baptist)

Mary says Behold the maidservant of the Lord!. Let it be to me according to your word. Gabriel then departed.

This is how Mary learned that she was going to give birth to Jesus, the Son of God.

Brief information about Mary and the Archangel Gabriel included.

Sources
The Bible
Wikipedia