The Kitáb-i-Aqdas - The Most Holy Book
Index term: two Most Great Festivals, under Holy Days
k110.
All Feasts have attained their consummation in the two Most Great Festivals, and in the two other Festivals that fall on the twin days -- the first of the Most Great Festivals being those days whereon the All-Merciful shed upon the whole of creation the effulgent glory of His most excellent Names and His most exalted Attributes, and the second being that day on which We raised up the One Who announced unto mankind the glad tidings of this Name, through which the dead have been resurrected and all who are in the heavens and on earth have been gathered together. Thus hath it been decreed by Him Who is the Ordainer, the Omniscient.
n138.
All Feasts have attained their consummation in the two Most Great Festivals, and in the two other Festivals that fall on the twin days
This passage establishes four great festivals of the Bahá'í year. The two designated by Bahá'u'lláh as "the two Most Great Festivals" are, first, the Festival of Ridván, which commemorates Bahá'u'lláh's Declaration of His Prophetic Mission in the Garden of Ridván in Baghdád during twelve days in April/May 1863 and is referred to by Him as "the King of Festivals" and, second, the Báb's Declaration, which occurred in May 1844 in Shíráz. The first, ninth and twelfth days of the Festival of Ridván are Holy Days (Q&A 1), as is the day of the Declaration of the Báb.
The "two other Festivals" are the anniversaries of the births of Bahá'u'lláh and the Báb. In the Muslim lunar calendar these fall on consecutive days, the birth of Bahá'u'lláh on the second day of the month of Muharram 1233 A.H. (12 November 1817), and the birth of the Báb on the first day of the same month 1235 A.H. (20 October 1819), respectively. They are thus referred to as the "Twin Birthdays" and Bahá'u'lláh states that these two days are accounted as one in the sight of God (Q&A 2). He states that, should they fall within the month of fasting, the command to fast shall not apply on those days (Q&A 36). Given that the Bahá'í calendar (see notes 26 and 147) is a solar calendar, it remains for the Universal House of Justice to determine whether the Twin Holy Birthdays are to be celebrated on a solar or lunar basis.