Sometimes informally called "Buddha's birthday," it actually encompasses the
birth, enlightenment Nirvana, and passing (Parinirvana) of Gautama Buddha.

In Mahayana Buddhist traditions, the holiday is known by its Sanskrit name,
वैशाख Vaiśākha, and derived variants of
it. (see link for rest of story)

http://what-buddha-said.net/drops/IV/Wesak_2009.htm