What astronomical event traditionally signals the beginning of Ramadan?

A crescent moon sighting traditionally signals the beginning of Ramadan.

The Islamic calendar is based on lunar months, so the start of Ramadan is traditionally established by seeing the young crescent moon after sunset at the end of Sha'ban. The month ends in the same way, leading to the beginning of Shawwal and Eid al-Fitr.

Different Muslim communities and authorities may use local sightings, reports from other regions, or astronomical calculations. These approaches can occasionally produce different announced dates, usually by one day.

The crescent used for this purpose is the thin new moon visible soon after sunset, not the full moon. This lunar method is why Ramadan and the two Eids move through the Gregorian calendar from year to year.