The Collaborative International Dictionary of English v.0.48:
Lady Day \La"dy` Day`\ (d[=a]) n. The day of the annunciation of the Virgin Mary, March 25. See Annunciation. [1913 Webster]WordNet (r) 3.0 (2006):
Lady Day n 1: a festival commemorating the announcement of the Incarnation by the angel Gabriel to the Virgin Mary; a quarter day in England, Wales, and Ireland [syn: Annunciation, Lady Day, Annunciation Day, March 25]