A purely lunar year is about eleven days short of a solar one, so Ramadan drifts backwards through the calendar and works its way round every season in about thirty-three years.

Twelve lunations come to roughly 354.37 days against the solar year's 365.24. The Islamic calendar keeps the Moon and lets the seasons slide, which is why Ramadan falls in high summer for a while and in midwinter a decade and a half later — a fasting day in Norway can run nineteen hours or nine depending on which part of the cycle you were born into. Most other lunar calendars refuse that drift and insert a leap month to stay in step with the Sun.