The Islamic calendar is anchored to a single act: sighting the new crescent moon (hilal) at sunset on the 29th day of a Hijri month. If the crescent is sighted, the next month begins the following day. If it is not sighted – whether because of cloud cover or because the moon has not yet emerged from the sun’s glare – the current month is completed as a 30-day month, and the new month begins the day after that. Every Hijri month is therefore either 29 or 30 days long, and the determination is made by direct observation rather than fixed calculation.
The instruction is given by the Prophet Muhammad (peace be upon him) in Sahih al-Bukhari 1841: “The Prophet (ﷺ) or Abul-Qasim said, “Start fasting on seeing the crescent (of Ramadan), and give up fasting on seeing the crescent (of Shawwal), and if the sky is overcast (and you cannot see it), complete thirty days of Sha’ban.” A parallel narration in Sahih Muslim 2401 preserves the same rule. The principle covers not only the start and end of Ramadan but the beginning of every Hijri month, including Shawwal (Eid al-Fitr) and Dhul-Hijjah (Hajj and Eid al-Adha).
The classical Sunni schools differ on whether a sighting in one country is binding on Muslims in another. The majority position is that a credible sighting reported through trustworthy channels is authoritative for those who receive the news, and that adjacent regions sharing the same horizon should follow a single sighting. A minority view holds that each region or country should determine the start of the month by its own local sighting (ru’yah baladiyyah). Both views are accepted within Sunni orthodoxy, and modern Muslim communities follow whichever practice their local scholarship has adopted.
Astronomical calculation plays a supporting but not decisive role. Modern computation can predict with high precision when the conjunction (the astronomical new moon) occurs and whether the crescent will be visible at sunset on a given evening. Many communities use calculation as a probability filter – rejecting sighting claims when the crescent is physically impossible to see – while still requiring an actual sighting before declaring the new month. A smaller number of communities have moved to calculation alone, particularly for advance planning of Ramadan and Eid al-Adha, though the majority position in Sunni Islam remains that confirmed sighting is the standard.
The dates shown on this calendar are astronomical approximations and are accurate enough for planning, but they are not authoritative for ritual purposes. The official start of Ramadan, the date of Eid al-Fitr, the beginning of Dhul-Hijjah, and the date of Eid al-Adha must come from the moon-sighting announcement of the authority you follow in your community. For most events, the calculated and observed dates agree; in some years they differ by one day. When they differ, the observed sighting takes precedence.