The Bahá'í Calendar started from the original Badíʿ Calendar, created by the Báb in the Kitabu'l-Asmá' and the Persian Bayán (5:3) in the 1840s. An early version of the calendar began to be implemented during his time. It used a scheme of 19 months of 19 days (19×19) for 361 days, plus intercalary days to make the calendar a solar calendar. The first day of the early implementation of the calendar year was Nowruz, while the intercalary days were assigned differently than the later Bahá'í implementation. The calendar contains many symbolic meanings and allusions including connections to prophecies of the Báb about the next Manifestation of God termed He whom God shall make manifest.
Bahá'u'lláh, the founder of the Bahá'í Faith, who claimed to be the one prophesied by the Báb, confirmed and adopted this calendar. Around 1870, he instructed Nabíl-i-A`zam, the author of The Dawn-Breakers, to write an overview of the Badíʿ calendar. In the Kitáb-i-Aqdas (1873) Bahá'u'lláh made Naw-Rúz the first day of the year, and also clarified the position of the Intercalary days to immediately precede the last month. Bahá'u'lláh set Naw-Rúz to the day on which the sun passes into the constellation Aries. Bahá'ís interpret this formula as a specification of the vernal equinox, though where that should be determined was not defined.
The calendar was first implemented in the West in 1907.
The Bahá'í scriptures left some issues regarding the implementation of the Badíʿ calendar to be resolved by the Universal House of Justice before the calendar can be observed uniformly worldwide. On 10 July 2014 the Universal House of Justice announced provisions that will enable the common implementation of the Badíʿ calendar worldwide, beginning at sunset 20 March 2015, coinciding with the completion of the ninth cycle of the calendar (see below).
Before Naw-Rúz 2015
The Bahá'í Calendar in western countries was synchronized to the Gregorian calendar, meaning that the extra day of a leap year occurred simultaneously in both calendars. The intercalary days stretched from 26 February to 1 March, automatically including the Gregorian leap day. There were 4 intercalary days in a regular year, and 5 in a leap year.
The practice in western countries was to start the year at sunset on March 20, regardless of when the vernal equinox technically occurs.
For eastern countries where the Islamic lunar calendar was used, the Bahá'í Calendar synchronized with the Islamic Lunar calendar. For example, the births of The Báb and of Bahá'u'lláh were commemorated according to their corresponding lunar calendar dates, which were the 1st and 2nd days, respectively, of the month of Muharram. Thus, the commemoration of these anniversaries would drift backwards about 11 days each year, and could therefore be gradually celebrated at any season (spring, winter, autumn, summer) of the year.
From Naw-Rúz 2015
In 2014, the Universal House of Justice selected Tehran, the birthplace of Bahá'u'lláh, as the location to which the date of the vernal equinox is to be fixed, thereby "unlocking" the Badíʿ calendar from the Gregorian calendar. For determining the dates, astronomical tables from reliable sources are used.
In the same message the Universal House of Justice decided that the birthdays of the Báb and Bahá'u'lláh will be celebrated on "the first and the second day following the occurrence of the eighth new moon after Naw-Rúz" (also with the use of astronomical tables) and fixed the dates of the Bahá'í Holy Days in the Bahá'í Calendar, standardizing dates for Bahá'ís worldwide. By this decision, the Badíʿ calendar was "unlocked" from the Islamic lunar calendar, as the celebration of the birthdays of the Báb and Bahá'u'lláh are no longer connected to the month of Muharram, and therefore do not drift continually backward by about 11 days, from year to year.
These changes came into effect as of sunset on 20 March 2015.