The Birth of the Bab commemorates the birth in 1819, at Shiraz in Persia, of Siyyid Ali-Muhammad, who in 1844 declared himself the Bab and founded the Babi religion, understood by Baha'is as the herald of their own faith. It is one of the two most joyful anniversaries of the Baha'i year, the Twin Birthdays (or Twin Holy Days), the other being the birth of Baha'u'llah. By the Islamic lunar dates on which the two were born, their birthdays fell on consecutive days, and Baha'is keep them together on two successive days, set each year by a lunar reckoning tied to the new moon of an appointed month, so that in the Gregorian calendar they usually fall in October or November.
The day is one of the nine holy days on which work is suspended, and is observed with prayers, readings and joyful gathering. The keeping of the two births together, rather than on separate fixed dates, is itself significant to Baha'is, expressing the intimate link between the Bab and Baha'u'llah, the herald and the one he heralded, whose missions the faith regards as two aspects of a single divine purpose. The Twin Birthdays are among the happiest occasions of the Baha'i calendar.