The lunar calendar is about 11 days shorter than the solar calendar.
A purely lunar year contains twelve lunar months. Since a synodic month—the cycle from one new moon to the next—lasts about 29.53 days, twelve of them total roughly 354 days. A solar, or tropical, year is about 365.24 days, producing a difference of approximately 11 days.
The exact gap is often described as 11 to 12 days because lunar months and calendar years involve rounding, leap days, and different calculation methods. In the Islamic Hijri calendar, which is a purely lunar calendar, the ordinary year has 354 days and some years have 355.
Because the Islamic calendar does not add a leap month to keep pace with the seasons, its months move backward through the Gregorian calendar by about 10 or 11 days each year. Over roughly 33 solar years—about 34 lunar years—Islamic months cycle through all the seasons. This is why Eid al-Adha can occur in summer, winter, or any other season over time.