Why can Ramadan dates vary?

The Moon’s phase cycle averages about 29.5 days, while an Islamic calendar month is set as either 29 or 30 days. Communities look for the thin waxing crescent around the end of the current month, then apply the calendar method they follow.

A new moon is not the same as a visible crescent. Visibility depends on the Moon’s position relative to the Sun, the time and place of observation, the horizon, and weather. NASA notes that the Moon’s visibility changes with its phase, season, and location on Earth.

Communities also differ on whether they accept a local sighting, a sighting reported elsewhere, or a calculated calendar. Those are differences in the rule used to announce the month, not necessarily disagreement about the astronomical phase.

Use the date on this site to plan ahead, then follow the announcement from the mosque or religious authority you rely on. A difference of one day between locations is possible.