What type of calendar makes Eid al-Adha shift on the Gregorian calendar?

Eid al-Adha shifts on the Gregorian calendar because the Islamic calendar is a lunar calendar. Its date is fixed as 10 Dhu al-Hijjah in the Hijri calendar, but lunar years are shorter than solar years.

An Islamic calendar year has 12 lunar months and lasts about 354 or 355 days, roughly 10 to 12 days less than a Gregorian year. As a result, Eid al-Adha moves backward through the Gregorian seasons, eventually occurring at different times of the year. It does not remain tied to a particular month such as June or December.

The exact Gregorian date can also vary between countries because Islamic months may begin according to local crescent-moon sightings or different official calculations. This is why communities can occasionally celebrate Eid on different Gregorian dates even while observing the same Islamic date. The Julian calendar is not responsible for the shift; it is an older solar calendar.