Why Islamic Dates Move About 11 Days Earlier Each Year

Ramadan moves every year because the Hijri calendar is purely lunar. Its 12 months follow the phases of the moon, and 12 lunar months add up to about 354 days. The Gregorian year follows the sun and lasts about 365 days. The Islamic year therefore ends about 11 days before the solar year does, and every Islamic date, Ramadan and both Eids included, arrives about 11 days earlier on the Gregorian calendar than it did the year before.

Key Takeaways

• 12 lunar months last about 354.37 days; a solar year lasts about 365.24 days. The gap is about 10.9 days.
• Year to year, a given Hijri date moves 10 to 12 days earlier on the Gregorian calendar, depending on month lengths and leap years.
• Over about 33 years, Ramadan passes through every season and returns to roughly the same Gregorian dates.
• The drift is by design: the Hijri calendar adds no leap month to keep its months in one season.

The arithmetic: 354 days against 365

The size of the shift comes from two astronomical numbers.

A lunar month runs from one new moon to the next. NASA's eclipse website gives the mean synodic month as 29.53059 days, about 29 days, 12 hours and 44 minutes. Individual months can run several hours longer or shorter than that average. A calendar month has to contain whole days, and the moon's cycle is about 29.5 days, so a Hijri month has either 29 or 30 days.

The solar year is the cycle of the seasons. Royal Museums Greenwich gives its length as 365.2422 days, which the Gregorian calendar approximates with its leap-year rule.

Put the two side by side:

That 10.875 days is the "about 11 days" people quote. The US Naval Observatory puts it the same way on its Islamic calendar page: 12 lunar months are, on average, 11 days shorter than the civil year.

The Quran describes this structure directly. In Surah At-Tawbah 9:36, a translation of the meaning (Saheeh International) begins: "Indeed, the number of months with Allah is twelve [lunar] months." In Surah Al-Baqarah 2:189, the crescent moons are described as "measurements of time for the people and for hajj."

A real Hijri year is a whole number of days, so it is either 354 or 355 days long. The sister post on how many days are in a Hijri year explains how those years are distributed.

Why the shift is 10, 11 or 12 days in practice

The 10.875-day figure is an average. The actual shift for a given date depends on two things:

• Hijri year length. A 355-day Hijri year shifts dates by one day less than a 354-day year.
• Gregorian leap years. If the stretch between two Ramadans includes 29 February, the Gregorian side gains a day, and the shift grows by one.

The calculated Umm al-Qura calendar used by the site's converter shows all three outcomes. Ramadan 1458 begins on 20 October 2036 and Ramadan 1459 on 10 October 2037, a shift of 10 days. Ramadan 1461 begins on 19 September 2039 and Ramadan 1462 on 7 September 2040, a shift of 12 days, because that span includes 29 February 2040. Most years fall in between, at 11.

A worked example: Ramadan through the seasons

The table below lists the first day of Ramadan in the calculated Umm al-Qura calendar for a run of years. Saudi Arabia uses this calculated calendar, which R.H. van Gent of Utrecht University describes in his introduction to the Umm al-Qura calendar. The same page notes that the dates different Muslim communities observe can differ by one, two or even three days, so treat these as planning dates, not announcements.

Method: the dates in this post come from the Umm al-Qura tables in the open-source, MIT-licensed hijri-converter library (the @tabby_ai/hijri-converter TypeScript port), the same data the site's converter uses. Weekdays and day counts were calculated from those dates.

The seasons named here are for the Northern Hemisphere. In the Southern Hemisphere they are reversed.

Read down the table and Ramadan walks backwards through the year: winter, autumn, summer, spring, then winter again. Ramadan 1481 begins four days before the Gregorian date on which Ramadan 1447 began.

Two details stand out:

• Two Ramadans in one Gregorian year. 2030 contains the start of Ramadan 1451 in January and the start of Ramadan 1452 in late December. This happens whenever the 11-day drift carries a date from the first days of a Gregorian year into its last days.
• Islamic dates start at sunset. A calendar that shows 1 Ramadan on Wednesday 18 February 2026 means the month begins at sunset on Tuesday 17 February, and the first fast is on Wednesday. The sister post on why the Islamic day begins at sunset covers this in detail.

The season matters for daily life because it sets the length of each fast. Ramadan near the June solstice means long fasting days in the north and short ones in the south; near December, the reverse. The post on why Ramadan fasting hours change with the seasons and latitude works through that effect.

The 33-year cycle

Because each Islamic date moves about 10.9 days earlier per Hijri year, it takes about 33.6 Hijri years, or about 32.6 Gregorian years, for a date to lap the whole solar year. The US Naval Observatory's calendar overview states that the cycle of twelve months "regresses through the seasons over a period of about 33 years."

Two rough equivalences fall out of the same numbers:

• 32 Gregorian years are close to 33 Hijri years. 32 solar years hold about 11,688 days; 33 lunar years hold about 11,694.
• 33 Gregorian years are close to 34 Hijri years. That is the pairing in the table above: 34 Hijri years separate Ramadan 1447 (2026) and Ramadan 1481 (2059), while 33 Gregorian years pass.

Neither pairing is exact, so a date never repeats perfectly. Each lap lands a few days off the previous one.

Over a lifetime the cycle comes round more than once. Someone who reaches 66 Gregorian years has seen Ramadan fall in every season about twice, and is about 68 in Hijri years.

Why not add a leap month?

Lunisolar calendars, such as the Hebrew and Chinese calendars, follow the moon but also keep pace with the solar year. The US Naval Observatory's overview of calendars explains that a common way to do this is to insert an extra month every few years, and that without such an adjustment the seasons steadily drift through the months.

The Islamic calendar does not. Its year is 12 lunar months, as in 9:36, and the verse that follows, 9:37, condemns nasi, the pre-Islamic practice of postponing the sacred months. The Hijri calendar has counted 12 lunar months with no leap month, so its months move freely through the seasons. The sister post on nasi and the end of the leap month gives the history and the main interpretations.

Surah Yunus 10:5 describes the moon's phases as a way "that you may know the number of years and account" (Saheeh International). The moving calendar is the result of counting that way, not an error in it.

Planning with a moving calendar

Because the drift is regular, you can estimate any Islamic date years in advance: take this year's Gregorian date and subtract about 11 days for each year ahead. For anything that matters, use a converter rather than mental arithmetic, since the 10-to-12-day variation adds up.

• Look up the calculated start of next Ramadan on the Ramadan holiday page. In the calculated calendar, Ramadan 1448 begins on Monday 8 February 2027, with the first night on the evening of Sunday 7 February.
• See the whole month day by day in the Ramadan 1448 calendar.
• Convert any other date, such as a birthday or an anniversary, with the Hijri date converter.

Calculated dates are for planning. Communities that rely on moon sighting may start Ramadan or Eid a day earlier or later, so confirm the date with your local mosque or the authority you follow.