Twin Holy Birthdays

[1][2][3] They are observed on the first and the second day following the occurrence of the eighth new moon after Naw-Rúz, as determined in advance by astronomical tables using Tehran as the point of reference.

[5] Prior to 2015 and a decision by the Universal House of Justice, these two holy days had been observed on the first and second days of Muharram in the Islamic lunar calendar in the Middle East, while other countries observed them according to the Gregorian calendar on 20 October (for the birth of the Báb) and 12 November (for the birth of Baháʼu'lláh).

[6][7] In October 2017 the Universal House of Justice sent a letter to "all who celebrate the Glory of God", on the meaning of Baháʼu'lláh's life and current Baháʼí activities, inspired by the 200th anniversary of his birth.

The notion of "twin Manifestations of God" is a concept fundamental to Baháʼí belief, describing the relationship between the Báb and Baháʼu'lláh.

Both are considered Manifestations of God in their own right, having each founded separate religions (Bábism and the Baháʼí Faith) and revealed their own holy scriptures.