Therefore the Lord himself will give you this sign: the virgin shall be with child, and bear a son, and shall name him Emmanuel, which means "God is with us!" (Is 7:10-14; 8:10)
https://bible.usccb.org/bible/readings/032523.cfm
The origin of the liturgical calendar is a complicated story. The Roman Empire used a solar calendar rather than a lunar calendar used by the Jews in Israel. Early Christians knew that the first Good Friday was on the 14th of Nisan . . . but wanted to know the corresponding date on the solar calendar. It was determined that it was March 25th. So the feast of the Annunciation was assigned to the date of the Lord's death . . . which is why nine months later we celebrate the Birth of the Messiah on December 25th.