Following the Council of Ephesus in 431, which affirmed Mary's role as Theotokos, the Western church settled on March 25 as the Feast of the Annunciation, nine months before Christmas. The date also became the medieval reckoning point for the start of the calendar year in England, known as Lady Day, used for centuries in legal and financial contracts.
For over a thousand years, much of Christian Europe considered March 25, not January 1, the true start of the new year. England and its colonies used this 'Annunciation Style' dating until 1752, meaning a document dated March 24, 1750 was followed the very next day by one dated March 25, 1751.
The date's placement nine months before Christmas was not accidental; early Christian theologians, working from Jewish traditions that great figures died on the same date they were conceived, worked backward from a presumed date of the Crucifixion to fix the Annunciation. Lady Day survives today as one of England's traditional 'quarter days,' when rents and leases were due, a rhythm of rural life that persisted well into the industrial era.