X-Day

Proper noun

 * 1)  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).
 * 2) An annual celebration in the  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.