How many days can Ramadan last, depending on crescent-moon sighting?

Ramadan can last 29 or 30 days, depending on the lunar month and crescent-moon sighting.

The Islamic calendar is lunar, so each month begins with the appearance of a new crescent according to the practice followed by a community. Ramadan ends when the next month, Shawwal, begins. If the crescent marking Shawwal is not seen after the 29th day, Ramadan continues for a 30th day; a lunar month cannot extend to 31 days.

This system explains why Ramadan moves through the Gregorian calendar. Twelve lunar months total about 354 or 355 days, roughly 10 to 11 days shorter than a solar year. As a result, Ramadan gradually shifts through winter, spring, summer, and autumn over the decades.

Muslim communities may begin and end Ramadan on different Gregorian dates because they can follow local sighting, reports from another region, astronomical calculations, or an official religious authority. Those differences concern the timing, not the permitted length: Ramadan is 29 or 30 days.