The crescent-visibility boundary differs from which line, causing Eid dates to vary locally?

The crescent-visibility boundary differs from the International Date Line, so Eid al-Adha can fall on different local dates.

Islamic months begin with the sighting or confirmation of the young crescent moon. Visibility depends on geography, weather, darkness, latitude and the moon’s position relative to the horizon. As a result, the first day of Dhu’l-Hijja—and therefore Eid on its 10th day—may be declared on different dates in different countries.

The International Date Line is a separate geographic boundary, running roughly through the Pacific Ocean, that separates consecutive calendar dates. It does not determine when the Islamic crescent is visible. The Prime Meridian, Equator and Tropic of Cancer likewise do not set Eid’s date.

This is why Eid al-Adha may occur on one of several Gregorian dates around the world even though it has a fixed Islamic-calendar date. Local religious authorities may follow direct observation, astronomical calculations or an official announcement from another country.