... celebrates the Annunciation, when (according to the Bible) the angel Gabriel informed the virgin Mary that she had been chosen to give birth to the Son of God. It's exactly nine months before Christmas Day.
Lady Day is one of the four English quarter days, on which servants were traditionally hired, school terms started, and rents were due. From 1155 to 1752 Lady Day was the first day of the new year; this is why the tax year starts on 6 April (this being 'old' Lady Day, before the twelve days were 'lost' on the adoption of the Gregorian calendar).
The other three English quarter days are Midsummmer Day (24 June), Michaelmas Day (29 September) and Christmas Day.
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