Like the Birth of the Bab, Baha'u'llah's birth is dated on the Islamic lunar calendar rather than fixed to the Baha'i solar calendar, falling on the second day of Muharram, immediately following the Bab's own birthday on the first day of that month, and consequently shifting position relative to the Gregorian calendar from year to year. Baha'i communities worldwide generally mark the two births together across consecutive days as the Twin Birthdays, one of the most widely and warmly celebrated periods in the Baha'i calendar.
The pairing reflects a central theological claim of the Baha'i Faith: that the Bab's mission and Baha'u'llah's mission form a single continuous unfolding of revelation, the Bab preparing the way for and explicitly foretelling the coming of the figure Baha'u'llah would later declare himself to be, so that commemorating their births together expresses a theological relationship as much as a calendrical coincidence.
Baha'u'llah's extensive later writings, composed across decades of exile following his public declaration, form the core scripture of the Baha'i Faith, and his birth anniversary accordingly holds a place of particular significance in the Baha'i devotional calendar, second in prominence only to the Ridvan festival marking his declaration itself.