1. X-Day - Noun
2. X-Day - Proper noun
(historical) 1 November 1945, the planned date for the Allied invasion of Kyushu in southwestern Japan in World War II (cancelled following Japan's unconditional surrender in September 1945).
An annual celebration in the Church of the SubGenius parody religion, falling on July 5 and commemorating the prophesied end of the world on that date in 1998, when alien beings from "Planet X" were to have invaded.
X-Day (plural X-Days)
Synonym of D-Day (“the date of any major event planned for the future”)