Annunciation– Fra Angelico (1426)
Here is a video that describes this scene in the painting above displayed in the Prado museum in Madrid.
In the first chapter of Luke The Annunciation(Luke 1:26-38) is described. We read how the angel Gabriel announced to Mary that she had been chosen to be the mother of the Christ, and how Mary answered, “Here I am, the handmaid of the Lord. Let it be to me as you have said.” It is reasonable to suppose that Our Lord was conceived immediately after this. Accordingly, since we celebrate His birth on 25 December, we celebrate the Annunciation nine months earlier, on 25 March.
For many centuries most European countries took 25 March, not 1 January, as the day when the number of the year changed, so that 24 March 1201 was followed by 25 March 1202. If you had asked a Christian of that time why the calendar year changed so awkwardly partway through a month, he would have answered: “Today we begin a new year of the Christian era, the era which began X years ago today when God was made man, when He took upon Himself a fleshly body and human nature in the womb of the Virgin.”