March 21

The Báb introduced a new calendar, known now as the Badi' or Bahá'í calendar … According to this calendar, a day is the period from sunset to sunset. In the Bayan, the Báb ordained the month of Ala' to be the month of fasting, decreed that the day of Naw-Ruz should mark the termination of that period, and designated Naw-Ruz as the Day of God. Bahá'u'lláh confirms the Badi' calendar wherein Naw-Ruz is designated as a feast. Naw-Ruz 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 and A 35). Hence Naw-Ruz 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-Ruz 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. 
(The Universal House of Justice, ‘Notes’ section of ‘The Kitab-i-Aqdas’)