The Annunciation is kept on 25 March and the Nativity on 25 December, and the interval is not a coincidence but the reason: the tradition fixed the conception first and counted forward. Which date was reasoned from which is disputed — one line of argument put the crucifixion on 25 March and gave the incarnation the same day, on the principle that a perfect life begins and ends on one date. Either way the feast of the Annunciation is older than most of the sees in this dataset.
New Advent (Fathers and Catholic Encyclopedia) · Catholic Encyclopedia — The Feast of the Annunciation