Here is an entry from  Dr. Mac's Cultural Calendar

Feast of the Annunciation.  Since the Church decided sometime in the 4th century to observe the Nativity of Jesus Christ on December 25 (it never taught that he was actually born on that day), and full-term since pregnancy has always been rounded off to nine months, March 25 was the day on which by sacred tradition the Archangel Gabriel appeared to the Blessed Virgin to tell her of her election to be what the Greek Church calls the theotokos, the God-bearer.  The New Year began on that date in England until the middle of the 18th century.  In England, it was called Lady Day.  All leases and contracts were dated from Lady Day.  Roman Church historian Dionysius Exiguus ca.500_550), in calculating his history of the Christian Church, took this day as the supposed date of the Annunciation.  March 25 afterward became the first day of the calendar year, until the Gregorian Calendar Reform of 1582 changed the day to January 1.

Malcolm