The Kitáb-i-Aqdas - The Most Holy Book
Index term: 'Alá', month of
We have enjoined upon you fasting during a brief period
Fasting and obligatory prayer constitute the two pillars that sustain the revealed Law of God. Bahá'u'lláh in one of His Tablets affirms that He has revealed the laws of obligatory prayer and fasting so that through them the believers may draw nigh unto God.
Shoghi Effendi indicates that the fasting period, which involves complete abstention from food and drink from sunrise till sunset, is

. . . essentially a period of meditation and prayer, of spiritual recuperation, during which the believer must strive to make the necessary readjustments in his inner life, and to refresh and reinvigorate the spiritual forces latent in his soul. Its significance and purpose are, therefore, fundamentally spiritual in character. Fasting is symbolic, and a reminder of abstinence from selfish and carnal desires.

Fasting is enjoined on all the believers once they attain the age of 15 and until they reach the age of 70 years.
A summary of the detailed provisions concerning the law of fasting and of the exemptions granted to certain categories of people is contained in the Synopsis and Codification, section IV.B.1.-6. For a discussion of the exemptions from fasting see notes 14, 20, 30 and 31.
The nineteen-day period of fasting coincides with the Bahá'í month of 'Alá', usually 2-20 March, immediately after the termination of the Intercalary Days (see notes 27 and 147), and is followed by the feast of Naw-Rúz (see note 26).
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.
Let the days in excess of the months be placed before the month of fasting.
The Badí' calendar is based on the solar year of 365 days, 5 hours, and 50 odd minutes. The year consists of 19 months of 19 days each (i.e. 361 days), with the addition of four extra days (five in a leap year). The Báb did not specifically define the place for the intercalary days in the new calendar. The Kitáb-i-Aqdas resolves this question by assigning the "excess" days a fixed position in the calendar immediately preceding the month of 'Alá', the period of fasting. For further details see the section on the Bahá'í calendar in The Bahá'í World, volume XVIII.
Question: Should a person wish to fast at a time other than in the month of 'Alá', is this permissible or not; and if he hath vowed or pledged himself to such a fast, is this valid and acceptable?
Answer: The ordinance of fasting is such as hath already been revealed. Should someone pledge himself, however, to offer up a fast to God, seeking in this way the fulfilment of a wish, or to realize some other aim, this is permissible, now as heretofore. Howbeit, it is God's wish, exalted be His glory, that vows and pledges be directed to such objectives as will profit mankind.