Why Does the Hijri Calendar Have 12 Months and No 13th Leap Month?

The short answer

The Hijri calendar keeps a fixed twelve lunar months and never inserts a thirteenth. Calendars that do add an occasional extra month — the luni-solar family — use it to drag their lunar months back into step with the solar year, a technique called intercalation. The Islamic calendar is purely lunar, so it skips that correction and drifts against the seasons by roughly 11 days a year, as the U.S. Naval Observatory describes.

Three ways to build a calendar

The Naval Observatory's introduction to calendars separates them cleanly:

Solar — tracks the sun and the seasons, ignoring the moon's phases. The Gregorian calendar is solar; its months are bookkeeping units, not lunar months.
Luni-solar — months follow the moon, but an extra month is inserted every few years so the year stays locked to the seasons. The Hebrew calendar is the familiar example.
Purely lunar — months follow the moon and nothing is inserted. Twelve lunar months are what they are: about 354 days.

Summary: the "13th month" is a repair tool for luni-solar calendars, and only those calendars need it.

Why the mismatch exists at all

A lunar month — new moon to new moon — averages a little over 29.5 days, which is why Hijri months run 29 or 30 days and alternate irregularly.
Twelve of them come to about 354 days.
A solar year is about 365.24 days.
The gap is roughly 11 days per year, and a luni-solar calendar closes it by adding a whole month every two or three years.

Without that extra month, the lunar year simply rotates against the solar one.

The historical switch people are asking about

Searches about moving "from 13 months to 12" usually point at one thing: intercalation was practiced in pre-Islamic Arabia, and the Islamic calendar as it was formalized fixed the year at twelve months and dropped the extra one. The practical consequence is the part worth remembering — the calendar became a pure lunar count, with no mechanism tying any month to a season.

It is also worth separating two different "13s" that get mixed together:

A 13th intercalary month, which is a calendar-design device.
A 13th lunation in a Gregorian year, which simply happens sometimes because 12 lunar months are shorter than 12 Gregorian ones.

Summary: twelve months is a design decision about intercalation, not an error in counting moons.

What the no-leap-month rule actually changes

The visible effects are all about drift:

Ramadan and Dhu al-Hijjah move through the seasons. Over roughly three decades, a given Hijri month works its way through the whole solar year and back.
A Hijri date never implies weather. A pilgrimage in Dhu al-Hijjah can fall in a hot part of the year or a mild one depending on the decade — which is why Makkah's weather by season is worth checking against Gregorian dates, not month names.
Anniversaries shift. A Hijri birthday drifts about 11 days earlier each Gregorian year, and about 33 Hijri years pass for every 32 Gregorian ones.
Planning windows move. Which months are busy or restricted changes year to year, as whether you can do Umrah any time of year lays out.

Why dates still need an official source

Because there is no leap month, every Hijri date depends on where each month begins — and that is settled either by calculation or by crescent sighting. Saudi Arabia publishes its civil dates through the Umm al-Qura calendar, which is why a printed almanac and a local announcement can disagree by a day even though both use twelve months. The underlying mechanics are set out in how the Hijri calendar works.

Summary: twelve months and no intercalation is what makes the Hijri calendar purely lunar — the 11-day annual drift is the direct, intended consequence.