The Bahá'í calendar , also called the Badí‘ calendar or Wondrous calendar ( badí‘ means wondrous or unique ),  is a solar calendar with years composed of 19 months of 19 days each (361 days) plus an extra period of " Intercalary Days ". Years begin at Naw-Rúz, on the day of the vernal equinox in Tehran, Iran, coinciding with March 20 or 21.
The year 174 BE started on the day of the vernal equinox in 2017, that is on 20 March 2017.
The Bahá'í calendar started from the original Badí‘ 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 Badí' 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 Badi' 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 Badi' 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 Badí' 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 Badí' 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.
Years in the Bahá'í calendar are counted from Thursday 21 March 1844, the beginning of the Bahá'í Era or Badí‘ Era (abbreviated BE or B.E. ).  Year 1 BE thus began at sundown 20 March 1844. Using the Bahá'í names for the weekday and month, day one of the Bahá'í Era was Istijlál (Majesty), 1 Bahá (Splendour) 1 BE. As detailed below, the names of the Bahá'í months and days reflect attributes of God.  William McElwee Miller's polemical work against the Bahá'í Faith claims that the date the calendar was to begin was 1850 CE though most sources agree the date was with the Declaration of the Bab to Mullá Husayn, on the evening of 22 May 1844 CE. (then the 7th of ‘Aẓamat)  The day is one of the Bahá'í holy days, remembered on the 8th day of ‘Aẓamat each year. The moment of the Declaration is celebrated two hours after the opening sunset (in the evening of May 22 or 23).
The Bahá'í calendar is composed of 19 months, each with 19 days.  The Nineteen Day Fast is held during the final month of ‘Alá’, and is preceded by the intercalary days, known as Ayyám-i-Há. The month of fasting is followed by Naw-Rúz, the new year.
The names of the months were adopted by the Báb from the Du'ay-i-Sahar, a Ramadan dawn prayer by Imam Muhammad al-Baqir, the fifth Imam of Twelver Shi'ah Islam.   These month names are considered to be referring to attributes of God.
In the Persian Bayan the Báb divides the months in four groups, of three, four, six and six months respectively.  Robin Mirshahi suggests a possible link with four realms described in Bahá'í cosmology. 
The days of the month have the same names as the names of the month – the 9th day of the month for example is the same as the 9th month – Asmá, or "Names". In the following table, the Gregorian date indicates the first full day of the month. The month begins at sunset of the Gregorian date previous to the one listed, after which time that month's Nineteen Day Feast may be celebrated.
|Month|| Usual Gregorian dates |
(when Naw-Rúz coincides with 21 March) 
|Arabic name ||Arabic script||English name ||Additional meanings in authorized English translations of Bahá'í scripture |
|1|| 21 March |
– 8 April
|Bahá||بهاء||Splendour||glory, light, excellence|
|2|| 9 April |
– 27 April
|3|| 28 April |
– 16 May
|4|| 17 May |
– 4 June
|‘Aẓamat||عظمة||Grandeur||glory, majesty, dominion, greatness|
|5|| 5 June |
– 23 June
|Núr||نور||Light||radiance, brightness, splendour, effulgence, illumination|
|6|| 24 June |
– 12 July
|Raḥmat||رحمة||Mercy||blessing, grace, favour, loving kindness, providence, compassion|
|7|| 13 July |
– 31 July
|Kalimát||كلمات||Words||utterance, the word of God|
|8|| 1 August |
– 19 August
|Kamál||كمال||Perfection||excellence, fullness, consummation, maturity|
|9|| 20 August |
– 7 September
|Asmá’||اسماء||Names||titles, attributes, designations|
|10|| 8 September |
– 26 September
|‘Izzat||عزة||Might||glory, power, exaltation, honour, majesty, grandeur, strength, sovereignty, magnificence|
|11|| 27 September |
– 15 October
|Ma sh íyyat||مشية||Will||purpose, the primal will, the will of God|
|12|| 16 October |
– 3 November
|‘Ilm||علم||Knowledge||wisdom, divine knowledge, revelation|
|13|| 4 November |
– 22 November
|Qudrat||قدرة||Power||might, authority, dominion, celestial might, omnipotence, transcendent power, indomitable strength, all-pervading power, ascendancy, divine power|
|14|| 23 November |
– 11 December
|15|| 12 December |
– 30 December
|Masá’il||مسائل||Questions||principles, truths, matters, mysteries, subtleties, obscurities, intricacies, problems|
|16|| 31 December |
– 18 January
|Sh araf||شرف||Honour||excellence, glory|
|17|| 19 January |
– 6 February
|Sulṭán||سلطان||Sovereignty||king, lord, majesty, sovereign, monarch, authority, potency, the power of sovereignty, the all-possessing, the most potent of rulers|
|18|| 7 February |
– 25 February
|Mulk||ملك||Dominion||sovereignty, kingdom, realm, universe|
| 26 February |
– 1 March
|Ayyám-i-Há||ايام الهاء||The Days of Há|
|19|| 2 March |
– 20 March
( Month of fasting)
The number of the intercalary days is determined in advance to ensure that the year ends on the day before the next vernal equinox. This results in 4 or 5 intercalary days being added. These days are inserted between the 18th and 19th months, falling around the end of February in the Gregorian calendar. The number of days added is unrelated to the timing of the Gregorian leap year.
The Bahá'í week starts on Saturday, and ends on Friday.  Like Judaism and Islam, days begin at sunset and end at sunset of the following solar day. Bahá'í writings indicate that Friday is to be kept as a day of rest.  The practice of keeping Friday as a day of rest is currently not observed in all countries; for example, in the UK, the National Spiritual Assembly of the Bahá'ís confirmed it does not currently keep this practice. 
|Arabic Name ||Arabic Script||English Translation ||Day of the Week |
Also existing in the Bahá'í calendar system is a 19 year cycle called Váḥid and a 361 year (19×19) supercycle called Kull-i- Sh ay’ (literally, "All Things").  Each of the 19 years in a Vahid has been given a name as shown in the table below. 
The 10th Váḥid of the 1st Kull-i- Sh ay' started on 21 March 2015, and the 11th Váḥid will begin in 2034. 
The concept of a 19 year cycle has existed in some form since the 4th century BCE. The Metonic cycle represents an invented measure that approximately correlates solar and lunar markings of time and which appears in several calendar systems.
- Years in a Váḥid
|No.||Persian Name||Arabic Script||English Translation|
As the name Badí‘ ( wondrous or unique ) suggests, the Bahá'í calendar is indeed a unique institution in the history of human culture. Sociologist Eviatar Zerubavel notes that the 19-day cycle creates a distinctive rhythm which enhances group solidarity. (Zerubavel argues that the 19 day cycle is more properly defined as a week, rather than a month, because it bears "no connection whatsoever" to the lunar cycle.) Furthermore, by finding the closest approximation of the square root of the annual cycle, Bahá'ís "have managed to establish the most symmetrical relationship possible between the week and the year, which no one else throughout history has ever managed to accomplish."