Date of 1st Easter, given in Scripture, and completely revolved around the established feasts of the Jews. Tradition (Jewish and Early Christian) believed that the day a saint, a.k.a holy person, died was on the same day they were conceived. Therefore, first Good Friday roughly equivalent to March 25, hence the reason that the Annunciation is celebrated on that day every year. Fast forward 9 months, December 25th. Not taking over any pagan holiday, simple math and peity determined the day that the Church celebrates Jesus birth (even if Jesus was most likely not born on December 25)