The Kitáb-i-Aqdas - The Most Holy Book
Index term: of Bahá'í Faith, under Sovereignty
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.
n49.
The Lord hath ordained that in every city a House of Justice be established
The institution of the House of Justice consists of elected councils which operate at the local, national and international levels of society. Bahá'u'lláh ordains both the Universal House of Justice and the Local Houses of Justice in the Kitáb-i-Aqdas. 'Abdu'l-Bahá, in His Will and Testament, provides for the Secondary (National or Regional) Houses of Justice and outlines the method to be pursued for the election of the Universal House of Justice.
In the verse cited above, the reference is to the Local House of Justice, an institution which is to be elected in a locality whenever there are nine or more resident adult Bahá'ís. For this purpose, the definition of adult was temporarily fixed at the age of 21 years by the Guardian, who indicated it was open to change by the Universal House of Justice in the future.
Local and Secondary Houses of Justice are, for the present, known as Local Spiritual Assemblies and National Spiritual Assemblies. Shoghi Effendi has indicated that this is a "temporary appellation" which,

. . . as the position and aims of the Bahá'í Faith are better understood and more fully recognized, will gradually be superseded by the permanent and more appropriate designation of House of Justice. Not only will the present-day Spiritual Assemblies be styled differently in future, but they will be enabled also to add to their present functions those powers, duties, and prerogatives necessitated by the recognition of the Faith of Bahá'u'lláh, not merely as one of the recognized religious systems of the world, but as the State Religion of an independent and Sovereign Power.