In the Christian calendar, Lady Day is the Feast of the Annunciation (25 March) and the first of the four traditional Irish Quarter days and English quarter days.
In England, it was New Year's Day up to 1752 when, following the move from the Julian Calendar to the Gregorian Calendar, 1 January started to start the year. A vestige of this remains in the United Kingdom's tax year, which starts on 6 April, i.e. Lady Day adjusted for the lost days of the calendar change.
Lady Day was also a nickname of the singer Billie Holiday.