Nasi: Why the Islamic Calendar Has No Leap Month

Nasi' (Arabic al-nasī', "postponement") was a pre-Islamic Arabian practice of moving the sacred months from their places in the year. Classical sources describe it in two ways: swapping the sanctity of one month onto another, or inserting an extra month every few years so the calendar kept pace with the seasons. The Quran condemns it in Surah at-Tawbah 9:37, and at his Farewell Pilgrimage in 632 CE the Prophet ﷺ announced that time had returned to its original order. Since then the Islamic calendar has been purely lunar: 12 months, no leap month, with each month drifting through the seasons.

Key Takeaways

• Nasi' means "postponement". Before Islam it let people shift which months counted as sacred, and many exegetes say it also added a month roughly every three years.
• Quran 9:37 calls the postponing "an increase in disbelief" and says it made lawful what Allah made unlawful.
• At the Farewell Pilgrimage the Prophet ﷺ said time had come back to its original state, with a year of twelve months, four of them sacred.
• The result is a 354- or 355-day lunar year with no leap month, so Ramadan and Hajj move about 11 days earlier each solar year.
• Some Islamic calendars add a leap day, never a leap month.

What nasi' means

The term comes from the Quran, in the verse right after the one that fixes the number of months. Surah at-Tawbah 9:36 begins, in a translation of the meaning (Sahih International on quran.com), "Indeed, the number of months with Allāh is twelve [lunar] months," and adds that four of them are sacred.

The next verse names the practice that broke this order. Surah at-Tawbah 9:37 opens: "Indeed, the postponing [of restriction within sacred months] is an increase in disbelief." It goes on to say that those who did it made the month lawful one year and unlawful another, to match the number of months Allah made sacred while making lawful what Allah had made unlawful.

The verse keeps the count of four sacred months intact. What it condemns is moving them: keeping the right number while changing which months they were.

How the postponement worked

Muslim commentators and modern historians agree that nasi' shifted the sacred months. They differ on how. Two main accounts appear in the sources, and they are not mutually exclusive.

Swapping a sacred month for an ordinary one

The first account treats nasi' as a relabelling of months within a normal year. The abridged Tafsir Ibn Kathir on 9:37 explains that the idolaters would allow Muharram one year and treat Safar as sacred in its place, so that the total stayed at four. It names al-Qalammas of the Kinanah tribe as the first to do this, and describes a later member of the same line announcing the change to the crowds at Hajj. It also says they kept counting the months of the year by their usual names.

A hadith shows the effect on Safar. In Sahih al-Bukhari 1564, Ibn Abbas reports pre-Islamic beliefs about Umrah. The Arabic text says they "made Muharram Safar", which the English on sunnah.com renders as treating Safar as a sacred month. They said Umrah became permissible once "the month of Safar passes away". In other words, the sacredness of Muharram had been moved onto the month after it.

Adding a thirteenth month

The second account treats nasi' as intercalation, the same idea as a leap month in a lunisolar calendar. Daniella Talmon-Heller summarises this reading in a chapter on the founding of an Islamic lunar calendar (Edinburgh University Press, 2020). According to Muslim exegetes, she writes, the verse accuses the pagans of Arabia of "adding a month" "roughly once every three years", and as a result they "observed the hajj on the wrong days". She also notes that the 11th-century scholar al-Biruni suggested the Arabs learned intercalation from Jewish communities in the region.

The sources do not settle how regular any such leap month was. What the two accounts share is the outcome that matters for the calendar: the sacred months, and with them the pilgrimage, did not stay where they belonged.

Why a lunar calendar is tempted to add a month

The arithmetic explains why a leap month was attractive. NASA's page on the Moon's orbit gives the mean synodic month, from one new moon to the next, as 29.53059 days. Twelve of those make about 354.37 days.

The solar or seasonal year is about 365.25 days, the figure Encyclopaedia Britannica uses for the Islamic calendar. A 12-month lunar year therefore falls about 11 days short of the seasons each year. After three years the gap is roughly 33 days, a little more than one lunar month.

A calendar that wants its months tied to the seasons has to add a thirteenth month about every third year. Lunisolar calendars such as the Hebrew calendar handle the gap with leap months, as compared in the sister post on Islamic, Jewish and Chinese lunar calendars.

The Farewell Pilgrimage: "time has come back"

The end of the practice is tied to the Prophet's Farewell Pilgrimage. Britannica's entry on the Farewell Pilgrimage dates it to 632 CE, a few months before his death.

During the sermon on the Day of Sacrifice, the Prophet ﷺ spoke about the calendar. In Sahih al-Bukhari 4406, from the chapter on the Farewell Pilgrimage, Abu Bakra reports that he said: "Time has taken its original shape which it had when Allah created the Heavens and the Earth." The narration continues that the year is twelve months, four of them sacred: Dhu al-Qa'dah, Dhu al-Hijjah and Muharram in succession, and Rajab of Mudar, "between (the month of) Jumaida (ath-thania) and Sha'ban." He then asked the people which month it was, and confirmed that it was Dhu al-Hijjah.

Imam al-Bukhari also places the same narration under the explanation of 9:36, in Sahih al-Bukhari 4662, where it is translated "Time has come back to its original state." Placing the hadith there links the sermon directly to the verse on twelve months and, by extension, to the verse on nasi' that follows it.

The words "time has come back" describe the months returning to their original positions. The detail about Rajab being "between Jumada and Sha'ban" pins the month to its place in the sequence. Talmon-Heller notes that some interpreters regarded this detail "as a reference to the abolishment of the pre-Islamic practice of intercalation." She adds that Ibn Taymiyya saw the end of intercalation as restoring the correct calendar and the proper timing of the Hajj.

The result: a purely lunar calendar

From then on, Muslims counted 12 lunar months and nothing else. The Quran ties the months to the moon itself: Surah al-Baqarah 2:189 says the crescent moons "are measurements of time for the people and for ḥajj [pilgrimage]." Britannica's article on the Islamic calendar states that "No other leap days or months are intercalated," so the named months move through the whole seasonal year every 32.5 solar years.

That drift is easy to see in the dates of a single month. The table shows the calculated start of Ramadan in 1447 AH and then at five-year steps from 1455 AH.

Dates are calculated from the Umm al-Qura tables in the open-source hijri-converter library (@tabby_ai/hijri-converter) that this site uses. Local announcements based on moon sighting may differ by a day.

After 33 Hijri years, Ramadan is back in late February, about 32 Gregorian years later. Over a lifetime, Muslims fast Ramadan in every season. With nasi', the months were pulled back toward fixed seasons. Without it, they follow the moon alone. The sister post on why Islamic dates move each year covers the drift in more detail.

A leap day is not a leap month

Some readers see "leap year" in Islamic calendar tables and wonder whether nasi' survived in another form. It did not. Tabular (arithmetical) Islamic calendars add one day to Dhu al-Hijjah in 11 years of every 30, as Britannica describes, to keep the months in step with the moon, not the sun. Calendars based on sighting or astronomical calculation simply let a month run to 29 or 30 days as the moon requires. Neither adds a month, and neither moves a sacred month. The sister post on how many days are in a Hijri year explains these 354- and 355-day years.

Solar calendars used for civil purposes are a separate question. The Ma'ariful Qur'an commentary on 9:37 records that some scholars held intercalation impermissible under the verse. It disagrees: keeping dates by a lunar or solar calendar is permissible, it says, but Islamic rulings follow the lunar calendar, and intercalation in a reckoning that does not affect those rulings falls outside the prohibition. For the dates of worship, the four sacred months, Ramadan and Hajj stay on the lunar count.

Common questions

Was nasi' only about the sacred months?

The verse addresses the sacred months, and the hadith in Sahih al-Bukhari 1564 shows the sanctity of Muharram being moved onto Safar. The intercalation reading adds that the whole calendar, including the month of Hajj, was pushed out of place. Both accounts concern the sacred months; they differ on whether an extra month was inserted to do it.

Did the Hijri calendar exist when nasi' was abolished?

Not as a numbered era. The lunar months were in use, but counting years from the Hijra came later, under the caliph Umar. Britannica dates the introduction of the Hijrah era to 639 CE. Earlier years are numbered AH retrospectively. Projecting the tabular Islamic calendar back places 1 Dhu al-Hijjah 10 AH on 28 February 632 (Julian calendar), so the Farewell Pilgrimage of 632 falls in 10 AH. That date is calculated with the standard 30-year tabular scheme and is approximate, since the months were then set by sighting.

Where to go from here

To see the twelve months in order, with the four sacred months marked, open the Islamic months guide. To see where a month falls in the solar year, look up any date in the Hijri date converter or browse the Ramadan 1448 calendar. Questions about using solar calendars alongside the Hijri one in religious matters are best taken to a qualified scholar or your local mosque.