The Kitáb-i-Aqdas - The Most Holy Book
Index term: timing of, under Naw-Rúz (New Year); Holy Days
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.
n147.
The number of months in a year, appointed in the Book of God, is nineteen.
The Bahá'í year, in accordance with the Badí' calendar, consists of nineteen months of nineteen days each, with the addition of certain intercalary days (four in an ordinary year and five in a leap year) between the eighteenth and nineteenth months in order to adjust the calendar to the solar year. The Báb named the months after certain attributes of God. The Bahá'í New Year, Naw-Rúz, is astronomically fixed, coinciding with the March equinox (see note 26). For further details, including the names of the days of the week and the months, see the section on the Bahá'í calendar in The Bahá'í World, volume XVIII.
q35.
 
Question: Concerning Naw-Rúz.
Answer: The Festival of Naw-Rúz falleth on the day that the sun entereth the sign of Aries [the vernal equinox in the northern hemisphere], even should this occur no more than one minute before sunset.