Today's Mass was historically called the "golden Mass" and celebrated with special solemnity, because it focuses on the role of Mary in the Incarnation.
The first reading is the famous prophecy from Isaiah about the virgin who will conceive and bear a son. The Gospel is the Annunciation account.
Mary is a special Advent figure. The expectant mother is a sign to us of what our Advent waiting is all about: the coming of Christ, our Savior.
Today's first reading from Isaiah is the same one we had yesterday (for the Fourth Sunday of Advent). Ahaz, the king, had entered into political alliances in an attempt to save Israel from her enemies. But the prophet Isaiah was telling him not to trust in politics, but in God. Only God could deliver Israel. In refusing to ask for a sign, Ahaz was not being humble, but tricky. He was keeping his options open, so to speak, by refusing to trust the Lord.
Mary, instead, was completely committed to doing God's will. By her "yes" to the angel Gabriel, Mary totally surrendered to what God was asking of her. She didn't know exactly what would happen. It was very risky--in those days a woman in an irregular pregnancy could suffer severe penalties. What would Joseph think?
Mary turned all those worries over to God. And God made it all work out. Whatever problems we face, God will help us also to work them out, if we turn to him in trust.