The Kitáb-i-Aqdas - The Most Holy Book
Index term: Universal House of Justice to fill in details, under Calendar, Bahá’í­
n26.
and at its close have designated for you Naw-Rúz as a feast
The Báb introduced a new calendar, known now as the Badí­' or Bahá’í­ calendar (see notes 27 and 147). According to this calendar, a day is the period from sunset to sunset. In the Bayán, the Báb ordained the month of 'Alá' to be the month of fasting, decreed that the day of Naw-Rúz should mark the termination of that period, and designated Naw-Rúz as the Day of God. Bahá’u’lláh confirms the Badí­' calendar wherein Naw-Rúz is designated as a feast.
Naw-Rúz is the first day of the new year. It coincides with the spring equinox in the northern hemisphere, which usually occurs on 21 March. Bahá’u’lláh explains that this feast day is to be celebrated on whatever day the sun passes into the constellation of Aries (i.e. the vernal equinox), even should this occur one minute before sunset (Q&A 35). Hence Naw-Rúz could fall on 20, 21, or 22 March, depending on the time of the equinox.
Bahá’u’lláh has left the details of many laws to be filled in by the Universal House of Justice. Among these are a number of matters affecting the Bahá’í­ calendar. The Guardian has stated that the implementation, worldwide, of the law concerning the timing of Naw-Rúz will require the choice of a particular spot on earth which will serve as the standard for the fixing of the time of the spring equinox. He also indicated that the choice of this spot has been left to the decision of the Universal House of Justice.
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.