The Annunciation is the first Joyful Mystery of the Holy Rosary.
The Angel Gabriel appeared to Mary at Nazareth, to inform her that God had chosen her to be the mother of His onlybegotten Son.
Gabriel addressed her saying, "Hail, full of Grace, the Lord is with thee. Blessed art thou among women."
When she heard this she was troubled at his word, and kept pondering what manner of greeting this might be.
And the Angel said to her, "Do not be afraid, 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 will give Him the throne of David His father, and he shall be king over the House of Jacob forever, and of His kingdom there shall be no end."
Mary pondered his request for some considerable time and finally said, "Behold the handmaid of the Lord, be it done to me according to thy word."
The tradition that Jesus was conceived and also died on 25th March is found in the writings of Hippolytus at the beginning of the third century. It is possible that it determined the date of Christmas, being nine months before December 25.
Also, according to Benedict XIV, there was an ancient tradition that 25th. March was the actual day.