How many days can the Islamic month of Ramadan last?

Ramadan can last 29 or 30 days, because the Islamic calendar is based on lunar months.

A lunar month begins when the new crescent is observed, or according to a calculated calendar used by some communities. If the crescent marking the next month is seen after the 29th day, Ramadan ends; otherwise, the month reaches 30 days.

This is why Ramadan moves through the seasons in the Gregorian calendar. The Islamic calendar has 12 lunar months and is about 10 to 12 days shorter than the solar year. Local moon-sighting decisions can also mean that Muslim communities begin or end Ramadan on different Gregorian dates.

Ramadan is followed by Shawwal, whose first day is Eid al-Fitr. The festival therefore begins immediately after the fasting month ends, whether Ramadan lasted 29 or 30 days.