This holy day is celebrated in the RC, Eastern Orthodox, Lutheran and High Anglican Churches as part of the Church Calendar.
Reading from the Synaxarion:
Six months after John the Forerunner’s conception, the Archangel Gabriel was sent by God to a city of Galilee named Nazareth unto Mary a Virgin, who according to the tradition handed down by the Fathers, she had been betrothed to Joseph, of the house of David, for four months and soon was to be wed.
Coming to her the Archangel Gabriel declared: “Rejoice, thou are Full of Grace, for the Lord is with thee: blessed art thou among women.” But she was greatly troubled at this and considered in her mind what sort of greeting this might be.
And the angel said to her, “Do not be afraid, Mary, for you have found favor with God. Behold, you will conceive in your womb and bear a son, and you shall call his name Emmanuel. He will be great, and will be called the Son of the Most High; and the Lord God will give to him the throne of his father David, and he will reign over the house of Jacob for ever; and of his kingdom there will be no end.”
And Mary said to the angel, “How shall this be, since I have no husband as of yet?” And the angel said to her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy, the Son of God.
After some consideration, and turmoil of her soul, the Virgin, having full assurance that this ineffable condescension and mighty dispensation was to take place said “Behold the handmaid of the Lord; be it unto me according to thy word.”
And so this fulfills the prophecy of Isaiah 7:14:
Ahaz (Jehoahaz), the son of Jotham, the son of Uzziah who was king of Judah, and the son of Rezin, King of Aram (Syria), and Pekahiah who was the son of Remaliah, king of Israel. But Ahaz was a blasphemer and idolator & wanted to march up to Jerusalem and subdue it.
11 And the Lord said to Ahaz, “Ask the Lord your God for a sign, whether in the deepest depths or in the highest heights. It does not matter what it is.”
12 Ahaz said, “I will not ask; as I will not put the Lord to the test.”13 Then Isaiah the prophet said, “Hear now, you house of David! Is it not enough to try the patience of humans? Now you also try the patience of my God?
14 “Therefore”, said the prophet, “the Lord himself will give you a sign and it will be that a virgin will conceive and give birth to a son, and she will call him Emmanuel. “(God is with us).
15 “And this son”, Isaiah warned, “will fast & eat sour curds and honey so that he knows enough to reject the wrong and choose the right, and the land of the two kings that now you dread will be laid waste.”