Luke Chapter 1
To a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin's name was Mary. And the angel being come in, said unto her: Hail, full of grace, the Lord is with thee: blessed art thou among women. Who having heard, was troubled at his saying, and thought with herself what manner of salutation this should be. And the angel said to her: Fear not, 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 shall give unto him the throne of David his father; and he shall reign in the house of Jacob for ever. And of his kingdom there shall be no end. And Mary said to the angel: How shall this be done, because I know not man? And the angel answering, said to her: The Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God. And behold thy cousin Elizabeth, she also hath conceived a son in her old age; and this is the sixth month with her that is called barren: Because no word shall be impossible with God.
And Mary said: Behold the handmaid of the Lord; be it done to me according to thy word. And the angel departed from her.
Catholic Encyclopedia: The Feast of the Annunciation
All Christian antiquity (against all astronomical possibility) recognized the 25th of March as the actual day of Our Lord's death. The opinion that the Incarnation also took place on that date is found in the pseudo-Cyprianic work "De Pascha Computus", c. 240. It argues that the coming of Our Lord and His death must have coincided with the creation and fall of Adam. And since the world was created in spring, the Saviour was also conceived and died shortly after the equinox of spring. Similar fanciful calculations are found in the early and later Middle Ages, and to them, no doubt, the dates of the feast of the Annunciation and of Christmas owe their origin.