Ridvan is the biggest Holy Day in the Baha'i faith, commemorating the commencement of Baha'u'llah's prophethood. The twelve day festival begins at sunset on April 20th and lasts until sunset, May 2nd. On the first, ninth and twelfth days of Ridvan, work is suspended. Ridvan means paradise, and is named for a garden encountered by the prophet during his travels.

Baha'u'llah

Related Resources:

Baha'i
Baha'i faith at Alternative Religions.

Recommended Reading:

Baha'i History
A thorough history of the Baha'i faith, including early Babism