Bahá'u'lláh designated the Festival of Ridván to be "the Most Great Festival". For a period of 12 days starting on April 21st, Bahá'ís around the world will celebrate Bahá'u'lláh's public declaration of His mission, an event which took place on the eve of His departure from Baghdád for Constantinople. He specified that the first, ninth and the twelfth days of this Festival should be celeberated and be observed as Holy Days.
The significance of Ridván for Bahá'ís can't be overstated. In 1844, the Báb had arisen to proclaim the coming of a great Messenger from God, the Promised One of all religions. During His six-year ministry, which culminated in His public execution on July 9, 1850, the Báb called the people of Persia to purify themselves in preparation for the arrival of "He whom God shall make manifest." Bahá'u'lláh, one of the Báb's foremost followers, was imprisoned in 1853 on false charges. While in prison, He experienced a revelation from God in which He learned that He was to be that Promised One. But upon His release from prison a few months later, He told no one of this experience. Indeed, for ten years, the entire duration of His exile in Baghdád, He kept silent on this matter. Even so, His character, wisdom, and deep spiritual insight affected all who came into contact with Him.
His growing influence prompted the authorities to seek to move Him to another place. Baghdád was an important crossroads at that time, and it was feared that the new religion might be spread far and wide if He were permitted to stay there, coming into contact with travelers from all quarters. Arrangements were therefore made to transfer Bahá'u'lláh and His party to Constantinople.
On the eve of His departure, Bahá'u'lláh took up residence in a garden which has since become known to Bahá'ís as the Garden of Ridván. He spent 12 days there in preparation for the long journey ahead. Guests flowed into the garden, rich and poor, powerful and lowly, all paying respects to the great Personage whose influence had touched them all. Sometime during the midst of all this activity, Bahá'u'lláh declared to the gathered Bábís that He was the Promised One spoken of by the Báb.