Eid al-Fitr’s Gregorian date falls about 11 days earlier each successive year.
Eid is fixed on 1 Shawwal in the Islamic Hijri calendar, which is lunar rather than solar. A Hijri year is roughly 354 days, while a Gregorian year is about 365 days, creating an average difference of approximately 11 days. As a result, Eid moves backward through the Gregorian calendar and eventually occurs in every season.
The shift is an average, not an exact annual rule. The Islamic calendar’s months alternate between 29 and 30 days, and local moon sighting can make Eid’s announced date differ by a day between countries. The date can also appear to shift by 10, 11, or 12 days in particular years.
This movement is why Ramadan and Eid may occur in summer one decade and winter another. After roughly 33 Islamic years, the lunar calendar has cycled through nearly all the seasons. In rare cases, Eid can even occur twice within one Gregorian calendar year.