April 21

He [Baha'u'llah] became well-known in regard to these qualities before the Báb appeared. Then Bahá'u'lláh declared the Báb's mission to be true and promulgated His teachings. The Báb announced that the greater Manifestation would take place after Him and called the Promised One "Him Whom God shall make manifest," saying that nine years later the reality of His own mission would become apparent. In His writings He stated that in the ninth year this expected One would be known; in the ninth year they would attain to all glory and felicity; in the ninth year they would advance rapidly. Between Bahá'u'lláh and the Báb there was communication privately. The Báb wrote a letter containing three hundred and sixty derivatives of the root Baha. The Báb was martyred in Tabriz; and Bahá'u'lláh, exiled into Iraq in 1852, announced Himself in Baghdad. For the Persian government had decided that as long as He remained in Persia the peace of the country would be disturbed; therefore, He was exiled in the expectation that Persia would become quiet. His banishment, however, produced the opposite effect. New tumult arose, and the mention of His greatness and influence spread everywhere throughout the country. The proclamation of His manifestation and mission was made in Baghdad. He called His friends together there and spoke to them of God. 
- 'Abdu'l-Baha  (From a talk, 18 April 1912, New York; ‘The Promulgation of Universal Peace: Talks Delivered by 'Abdu'l-Bahá during His Visit to the United States and Canada in 1912’)