The Hijri calendar and Umm al-Qura, explained
By Dexaloop · Updated 27 October 2026 · Checked against the sources listed below
The Hijri calendar is a lunar calendar of 12 months, each 29 or 30 days long, so a Hijri year has about 354 days. Saudi Arabia's Umm al-Qura calendar is a calculated version of it: an astronomical rule, not a sighting, decides when each month begins. Other countries and communities may start a month on the evening the new crescent is actually seen, which can be a day later or earlier. That is why the same Hijri date can fall on different Gregorian days in different places.
How a lunar month works
A lunar month follows the Moon's phases, from one new moon to the next. NASA gives the average length: "The mean length of the synodic month is 29.53059 days." Individual months vary around that average by several hours.
A calendar month must be a whole number of days, so Hijri months are either 29 or 30 days. Where months follow sighting, a new month begins on the evening the thin new crescent (the hilal) is first seen. If it is not seen on the evening of the 29th day, the month runs to 30 days.
Worked example: why the Hijri year drifts
Twelve average lunar months come to 12 x 29.53059 = 354.37 days. So Hijri years are 354 or 355 days long.
A Gregorian year is 365 or 366 days. The difference is about 365.24 - 354.37 = 10.87 days a year. In practice, each Hijri month starts about 11 days earlier in the Gregorian calendar than it did the year before.
Keep going and the drift adds up. After about 365.24 / 10.87, or roughly 33.6 Gregorian years, a Hijri month has moved all the way around the seasons. That is why Ramadan can fall in summer for some years and in winter for others.
| Hijri calendar | Gregorian calendar | |
|---|---|---|
| Based on | Phases of the Moon | The solar year |
| Month length | 29 or 30 days | 28 to 31 days |
| Year length | 354 or 355 days | 365 or 366 days |
| Fixed to seasons | No, moves about 11 days a year | Yes |
What the Umm al-Qura calendar is
Umm al-Qura is the calendar the government of Saudi Arabia uses for official and administrative dates. The Utrecht University reference by R.H. van Gent describes it as "used by the government of Saudi Arabia for civil purposes." Its dates are calculated in advance, which is what makes it practical for printed calendars, contracts and government forms.
According to the same reference, since 1423 AH (March 2002) the rule has been this. On the 29th day of a month, the next day starts a new month if, seen from Mecca:
- the conjunction (the astronomical new moon) happens before sunset, and
- the Moon sets after the Sun.
If both are true, the month ends after 29 days. If not, it has 30 days. Note what the rule does not test: whether the crescent is actually visible. A very young Moon can set after the Sun and still be far too faint to see.
Van Gent also notes that the calendar is followed beyond Saudi Arabia, naming countries "such as Bahrain, Qatar and the United Arab Emirates," and many Muslim communities elsewhere through software and printed calendars.
Calculated dates versus sighting
For civil dates, Saudi Arabia uses the calculated calendar. The start of religious months such as Ramadan, Shawwal (Eid al-Fitr) and Dhu al-Hijja is announced separately in Saudi Arabia, after reports of the crescent. Van Gent's adjustment page says plainly that "the Umm al-Qura calendar is intended for civil purposes only," and lists cases where the announced start of a month differed from the calculated one.
The gap goes both ways. Because the Umm al-Qura rule does not require visibility, the first actual sighting can come later than the calculated date. Van Gent writes that it "can occur up to two days after the date predicted by the Umm al-Qura calendar." In other listed years, an announced start came a day before the calculated date.
The reference also records one striking effect of these adjustments. When a month was moved, a day could be counted twice: "both Friday 28 December and Saturday 29 December 2007 were reckoned as 19 Dhu ʾl-Ḥijja 1428 AH."
Why dates differ between countries
There is more than one accepted way to begin a month, and different countries and communities follow different ones. Broadly, they include:
- A calculated calendar, such as Umm al-Qura, fixed in advance.
- Local sighting: the month starts when the crescent is seen in that country.
- Following another authority: some communities abroad follow the Saudi calendar or announcements rather than their own sighting. Van Gent names several North American and European bodies that announced they would follow Umm al-Qura.
A crescent that is seen from one place on a given evening may not be seen from another, and different groups accept reports by different standards. Van Gent notes that around Ramadan and Dhu al-Hijja, dates observed by different groups can "differ by one, two or even three days." Each method has scholarly support, and this guide does not judge between them.
What this means for a printed calendar
A printed Hijri calendar can only show calculated dates, because sightings happen in real time. A good printed calendar marks religious dates such as Ramadan and the two Eids as expected dates. The official public holiday is fixed by the announcement in each country, which may move it by a day.
For planning, use the calculated date and allow at least a day of slack on either side. For anything that depends on the actual day, such as a holiday rota, wait for the official announcement where you live.
Common mistakes
- Treating a Hijri date as fixed to a Gregorian date. It moves about 11 days earlier every year.
- Assuming Umm al-Qura dates are the observed religious dates. They are calculated civil dates. Observed dates may differ.
- Using one country's dates for another. Neighboring countries can start Ramadan or Eid on different days.
Print a 2027 Saudi calendar with Hijri dates: print.dexaloop.com/calendars/2027/saudi-arabia/. Free, no sign-up, and it runs on your device.
Questions
What is the Umm al-Qura calendar?
It is a calculated Hijri calendar used by the government of Saudi Arabia for civil purposes. Its months begin by an astronomical rule based on Mecca, rather than on a reported sighting of the new crescent.
How long is a Hijri year?
Twelve lunar months of 29 or 30 days. With the average lunar month of about 29.53 days, that is about 354.4 days, so a Hijri year has 354 or 355 days.
Why does Ramadan move earlier every year?
The Hijri year is about 11 days shorter than the Gregorian year, so each Hijri month starts about 11 days earlier in the Gregorian calendar than it did the year before.
Why do countries start Ramadan on different days?
Communities use different methods: a calculated calendar, a local sighting of the crescent, or a sighting reported elsewhere. The crescent can be visible in one place a day before another, so the start can differ by a day, and occasionally more.
Can a printed Hijri calendar be wrong?
A printed calendar shows the calculated date. The date a community actually observes for Ramadan or Eid may be announced later and can differ by a day. Treat printed religious dates as expected dates and follow the local announcement.
Sources
- R.H. van Gent, Utrecht University: The Umm al-Qura Calendar of Saudi Arabia, introduction
- R.H. van Gent, Utrecht University: Umm al-Qura calendar, astronomical rules
- R.H. van Gent, Utrecht University: Adjustment of the Umm al-Qura calendar
- NASA Goddard: Eclipses and the Moon's Orbit (synodic month)
This guide is general information, not legal, tax or financial advice. Rules change; check the official source for your own case. Found a mistake? Email support@dexaloop.com.