Lunar calendar in the context of "Synodic month"

The Hijri calendar (Arabic: ٱلتَّقْوِيم ٱلْهِجْرِيّ, romanized: al-taqwīm al-hijrī), also known in English as the Islamic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to determine the proper days of Islamic holidays and rituals, such as the annual fasting and the annual season for the great pilgrimage. In almost all countries where the predominant religion is Islam, the civil calendar is the Gregorian calendar, with Syriac month-names used in the Levant and Mesopotamia (Iraq, Syria, Jordan, Lebanon and Palestine), but the religious calendar is the Hijri one.

This calendar enumerates the Hijri era, whose epoch was established as the Islamic New Year in 622 CE. During that year, Muhammad and his followers migrated from Mecca to Medina and established the first Muslim community (ummah), an event commemorated as the Hijrah. In the West, dates in this era are usually denoted AH (Latin: Anno Hegirae, lit. 'In the year of the Hijrah'). In Muslim countries, it is also sometimes denoted as H from its Arabic form (سَنَة هِجْرِيَّة, abbreviated ھ). In English, years prior to the Hijra are denoted as BH ("Before the Hijra").

Hajj (/hɑːdʒ/; Arabic: حَجّ, romanized: Ḥajj; also spelled Hadj or Haj) is an annual Islamic pilgrimage to Mecca, Saudi Arabia, the holiest city for Muslims. Hajj is a mandatory religious duty for capable Muslims that must be carried out at least once in their lifetime by all adult Muslims who are physically and financially capable of undertaking the journey, and of supporting their family during their absence from home.

In Islamic terminology, Hajj is a pilgrimage made to the Kaaba, the "House of Allah", in the sacred city of Mecca in Saudi Arabia. It is one of the Five Pillars of Islam, alongside Shahadah (oath that one believes there is no god but Allah), salat (prayer), zakat (almsgiving), and sawm (fasting during Ramadan). The Hajj is an annual practice when Muslim brotherhood is on display and their solidarity with fellow Muslim people and submission to God (Allah) is fulfilled. The Hajj is taken by Muslims to cleanse their souls of all worldly sins, which connotes both the outward act of a journey after death and the inward act of good intentions. The rites of pilgrimage are performed over five to six days, extending from the 8th to the 12th or 13th of Dhu al-Hijjah, the last month of the Islamic calendar. Because the Islamic calendar is lunar and the Islamic year is about eleven-twelve days shorter than the Gregorian year, the Gregorian date of Hajj changes from year to year. In 2024 AD (1445 AH), Dhu al-Hijjah extended from 7 June to 6 July and in 2025 AD (1446 AH), Dhu al-Hijjah was from 28 May to 25 June. In 2026 AD (1447 AH), Dhu al-Hijjah will extend from 18 May to 15 June; and in 2027 AD (1448 AH), Dhu al-Hijjah will extend from 24 May to 22 June.

A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) are more exact. The modern calendar year, as reckoned according to the Gregorian calendar, approximates the tropical year by using a system of leap years.

The term 'year' is also used to indicate other periods of roughly similar duration, such as the lunar year (a roughly 354-day cycle of twelve of the Moon's phases – see lunar calendar), as well as periods loosely associated with the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc.

A religious festival is a time of special importance marked by adherents to that religion. Religious festivals are commonly celebrated on recurring cycles in a calendar year or lunar calendar. The science of religious rites and festivals is known as heortology.

Within the Evangelical Lutheran branch of Christianity, "festival" is part of the ranking of feast days in the liturgical kalendar.

In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.

A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, afternoon, evening, and night. This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes.

A collection of sequential days is organized into calendars as dates, almost always into weeks, months and years. A solar calendar organizes dates based on the Sun's annual cycle, giving consistent start dates for the four seasons from year to year. A lunar calendar organizes dates based on the Moon's lunar phase.