Ridvan is a twelve day Baha'i festival celebrating both Baha'u'llah's revelation to close followers that he is the next Manifestation of God, whose coming the Bab predicted, and his preparations to travel from Baghdad to Istanbul. It stretches between April 21 and May 2.
The first (April 21), ninth (April 29), and twelfth (May 2) days of Ridvan are considered major holidays as per the dictates of Baha'u'llah. These days correspond to Baha'u'llah's move to Najibiyyih Garden, which Baha'u'llah called Ridvan, or Paradise, the day his family joined him at the Garden, and the day they all left the Garden to start journeying to Istanbul.
In 1863, Baha'u'llah was asked to move from Baghdad to Istanbul for political reasons. As the family packed up their belongings, Baha'u'llah relocated himself to nearby Najibiyyih Garden. This allowed him to meet with the many guests coming to say goodbye without interfering with the hasty packing.
Even though Baha'u'llah had received his vision of being the Manifestation of God in 1852, it was only on his first evening in the Garden that he shared knowledge of that vision with anyone. At the gardens, it was probably only a handful of followers, including his son, Abdu'l-Baha.
Ridvan marks the beginning of Baha'u'llah's exile from Baghdad, which he himself compared with Islam's Mohammad exile from Mecca.
Furthermore, there is a certain sense of completion in Ridvan. The Baha'i calendar groups years into 19 year spans called Vahids. The calendar started in 1844, the year that the Babannounced his own revelations, founding the Babi Faith out of which grew the Baha'i Faith. Thus, the events of Ridvan happened in the 19th year of the first Vahid.
Common CelebrationsLike other holidays, Baha'is have few expectations on how Ridvan should be celebrated. Commonly it's marked with community gatherings for prayers festivity on the first, ninth and twelfth days, and work is suspended for those days.
Elections to Baha'i assemblies are also commonly performed at this time.