Now in the sixth month the angel Gabriel was sent by God to a city of Galilee named Nazareth, to a virgin betrothed to a man whose name was Joseph, of the house of David. The virgin's name was Mary. And having come in, the angel said to her, "Rejoice, highly favored one, the Lord is with you; blessed are you among women!"
But when she saw him, she was troubled at his saying, and considered what manner of greeting this was. Then the angel said to her, "Do not be afraid, Mary, for you have found favor with God. And behold, you will conceive in your womb and bring forth a Son, and shall call His name JESUS. He will be great, and will be called the Son of the Highest; and the Lord God will give Him the throne of His father David. And He will reign over the house of Jacob forever, and of His kingdom there will be no end."
Then Mary said to the angel, "How can this be, since I do not know a man?"
And the angel answered and said to her, "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. Now indeed, Elizabeth your relative has also conceived a son in her old age; and this is now the sixth month for her who was called barren. For with God nothing will be impossible."
Then Mary said, "Behold the maidservant of the Lord! Let it be to me according to your word." And the angel departed from her.
Luke 1:26-38

On this day, the Church celebrates the Annunciation. According to Wikipedia,

When the calendar system of Anno Domini was first introduced by Dionysius Exiguus in AD 525, he assigned the beginning of the new year to March 25, since according to Christian theology, the era of grace began with the Incarnation of Christ.

Whenever I read the Biblical account, I am reminded of Hannah in 1 Samuel. The simple and profound faith of Hannah and Mary contrast sharply with the doubts of the two priests Eli and Zechariah. But isn't it amazing that the birth of John was announced in the Temple. Yet the birth of the Messiah was announced in an insignificant village to a young woman. Of course, God's ways are always best.
Nazareth was a small village of less than two thousand inhabitants. It was so insignificant, that Luke gives its location in Galilee, since many of his readers might not have known where to locate it or even heard of it.
In Judaism, a virgin was a young woman of fourteen or younger. Luke's word parthenos also means she had never had intercourse with a man, which she indicates herself in verses 34-35. As Mary was a woman, so young, and unmarried, she had no social status. The angels words were certainly startling. Highly favored one, the Lord is with you, blessed were not normal phrases used in greeting, especially to her. This teenage girl has joined the ranks of Noah (Genesis 6:8), Lot (Genesis 19:16, 19), and Moses (Exodus 33:13). God would be with her through all the upcoming ordeals. (Note: she did not highly favor herself, nor bless herself. But these were the gifts of God to her. If you want blessed, go to the Source, not the recipient.)

He will be great, and will be called the Son of the Highest; and the Lord God will give Him the throne of His father David. And He will reign over the house of Jacob forever, and of His kingdom there will be no end.
Luke 1:32-33

This language comes from God's promise to David in 2 Samuel 7:12-16:

When your days are fulfilled and you rest with your fathers, I will set up your seed after you, who will come from your body, and I will establish his kingdom. He shall build a house for My name, and I will establish the throne of his kingdom forever. I will be his Father, and he shall be My son. If he commits iniquity, I will chasten him with the rod of men and with the blows of the sons of men. But My mercy shall not depart from him, as I took it from Saul, whom I removed from before you. And your house and your kingdom shall be established forever before Me. Your throne shall be established forever.

And also from Isaiah 9:6-7:

For unto us a Child is born,
Unto us a Son is given;
And the government will be upon His shoulder.
And His name will be called
Wonderful, Counselor, Mighty God,
Everlasting Father, Prince of Peace.
Of the increase of His government and peace
There will be no end,
Upon the throne of David and over His kingdom,
To order it and establish it with judgment and justice
From that time forward, even forever.
The zeal of the Lord of hosts will perform this.

Mary's Son would be the Mighty God.
The angel told her, "For with God nothing will be impossible." When the Lord had told Abraham, "Is anything too hard for the LORD?" (Genesis 18:14), Sarah had laughed (v. 12-15). Mary showed greater faith than her ancestor, simply submitting to the will of God.