The Kitáb-i-Aqdas - The Most Holy Book
Index term: immediately follows Fast, under Naw-Rúz (New Year); Holy Days
k16.
O Pen of the Most High! Say: O people of the world! We have enjoined upon you fasting during a brief period, and at its close have designated for you Naw-Rúz as a feast. Thus hath the Day-Star of Utterance shone forth above the horizon of the Book as decreed by Him Who is the Lord of the beginning and the end. Let the days in excess of the months be placed before the month of fasting. We have ordained that these, amid all nights and days, shall be the manifestations of the letter Há, and thus they have not been bounded by the limits of the year and its months. It behoveth the people of Bahá, throughout these days, to provide good cheer for themselves, their kindred and, beyond them, the poor and needy, and with joy and exultation to hail and glorify their Lord, to sing His praise and magnify His Name; and when they end -- these days of giving that precede the season of restraint -- let them enter upon the Fast. Thus hath it been ordained by Him Who is the Lord of all mankind. The traveller, the ailing, those who are with child or giving suck, are not bound by the Fast; they have been exempted by God as a token of His grace. He, verily, is the Almighty, the Most Generous.
n25.
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).
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.