Which solar calendar's date for Eid al-Adha varies from year to year?

Eid al-Adha’s date in the Gregorian calendar varies from year to year.

Eid al-Adha is fixed in the Islamic Hijri calendar: it begins on 10 Dhu'l-Hijja, the final month of the Islamic year. The Hijri calendar is lunar, with months tied traditionally to the sighting of the crescent moon, while the Gregorian calendar is solar and follows the Earth’s cycle around the Sun.

A lunar year is roughly 10 to 12 days shorter than a Gregorian year. As a result, Eid al-Adha moves earlier by about a week and a half each year when converted into Gregorian dates. Over a cycle of roughly 32 or 33 years, it passes through every season.

A common mix-up is to assume that the festival date itself changes within the Islamic calendar. It does not: the festival remains on 10 Dhu'l-Hijja, although local moon-sighting practices can make the Gregorian date differ by a day between countries.