The Festivals of the Twin Birthdays or the Twin Holy Birthdays refers to two successive holy days in the Bahá’í calendar that celebrate the births of two central figures of the Bahá’í Faith. The two holy days are the Birth of the Báb on the first day of Muharram in the Islamic calendar and the Birth of Bahá’u’lláh on the second day of Muharram. In the Kitáb-i-Aqdas, Baha’u’llah has stated: “These two days are accounted as one in the sight of God.”
Prior to 2015 these two holy days had been observed on the first and second days of Muharram in the Islamic lunar calendar by the Baha’is in the Middle East, while other countries observed them according to the Gregorian calendar on October 20 for the birth of the Báb (1819) and November 12 for the birth of Bahá’u’lláh (1817).
With the approach of the bicentennial celebrations of the Bab and Baha’u’llah in a letter dated 10 July 2014 the Universal House of Justice made the decision to unite the Baha’i world in celebration of these holy birthdays and to fulfill Bahá’u’lláh’s vision for these two holy days being celebrated as one. They wrote: “How to satisfy the intrinsic lunar character of these blessed Days within the context of a solar calendar has hitherto been unanswered. We have decided that they will now be 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 Ṭihrán as the point of reference. This will result in the observance of the Twin Birthdays moving, year to year, within the months of Mashíyyat, ‘Ilm, and Qudrat of the Badí‘ calendar, or from mid-October to mid-November according to the Gregorian calendar.” Note: Naw-Rúz is celebrated on the day on which the vernal equinox occurs.
The notion of “twin Manifestations of God” is a concept fundamental to Bahá’í belief, describing the relationship between the Báb and Bahá’u’lláh. Both are considered Manifestations of God in their own right, having each founded separate religions (Bábism and the Bahá’í Faith) and revealed their own holy scriptures. To Bahá’ís, however, the missions of the Báb and Bahá’u’lláh are inextricably linked: The Báb’s mission was to prepare the way for the coming of Him whom God shall make manifest, who eventually appeared in the person of Bahá’u’lláh. For this reason, both the Báb and Bahá’u’lláh are revered as central figures of the Bahá’í Faith. A parallel is made between Bahá’u’lláh and the Báb as between Jesus and John the Baptist. However, in the Bahá’í Faith both the Báb and Bahá’u’lláh are considered Manifestations of God. Further cementing the bond between these two Manifestations is the celebration of their Twin Birthdays on two successive days.