Every other major lunar calendar inserts a leap month to keep up with the seasons. The Islamic calendar was explicitly forbidden to, and drifts a full year in every thirty-three.

Pre-Islamic Arabia used an intercalation called nasī' to keep the pilgrimage in a convenient season, and the Qur'an prohibits it as tampering. So the Hijri calendar is purely lunar, and Ramadan works its way through summer and winter over a lifetime — which is a real difference in the length of a fasting day, and a deliberate one.