Why Ramadan moves and Passover doesn't
If you have noticed Ramadan creeping earlier each year — summer, then spring, then winter, then back round — while Easter and Passover stay stubbornly in spring, you have spotted something real. Both calendars are built on the same moon. They differ because of one decision, made deliberately, about whether to correct for a gap in the arithmetic.
The eleven-day problem
A lunar month — new moon to new moon — averages 29.53 days. Twelve of them make 354.4 days. But the solar year, the one that governs the seasons and the harvest, is 365.24 days.
The gap is about eleven days a year. That is the whole thing. Every purely lunar calendar runs eleven days fast against the sun, every single year, forever.
Left alone, a festival fixed to a lunar date slides backwards through the seasons. Eleven days a year is a full month in three years, a season in about eight, and a complete circuit of the year in roughly 33.
Two answers to the same problem
The Hebrew calendar corrects
Roughly every third year an entire extra month — Adar II — is added, and the festivals snap back into place. Passover is tied to the spring barley harvest and to the equinox, so drifting out of spring would break the festival's meaning. The correction settles into a nineteen-year rhythm with seven leap years, though which nineteen-year rhythm is itself disputed.
The Islamic calendar refuses
The Hijri calendar has twelve lunar months and no correction at all, on purpose. Pre-Islamic Arabia had used an intercalation practice called nasī'; Qur'an 9:36–37, proclaimed at the Farewell Pilgrimage, abolished it. So Ramadan is not tied to a season and was never meant to be. It moves, permanently.
What that looks like
This is easier to watch than to read. The volvelle on the front page runs both calendars against a fixed solar ring: switch to Islamic mode, press run, and the marker walks steadily backwards through all four seasons. Switch to Hebrew mode and watch it drift the same way — then jump back when the thirteenth month lands.
Open the instrument and run it →
The consequences, which are not trivial
- Ramadan's difficulty changes over a lifetime. Fasting from dawn to sunset in high-latitude midsummer is a very different undertaking from fasting in midwinter. Observe for 33 years and you experience the whole range.
- Your Hijri and Gregorian birthdays diverge. Because the Hijri year is ~11 days shorter, a person's age in Hijri years slowly outruns their age in Gregorian ones — by roughly one year every 33.
- Passover keeps its meaning. The correction exists so that a spring festival stays a spring festival. That is not an accident of arithmetic; it is the point of the arithmetic.
Why anyone should care beyond the curiosity
This same machinery is what makes dating ancient events hard. If you want to know which Friday the crucifixion fell on, you need to know when Nisan began — and that depends on when the crescent was first sighted and whether that year took a thirteenth month. Both were human judgements, made locally, and neither is recorded.
So the eleven-day gap is not a footnote. It is the reason a two-thousand-year-old date is still argued over, and the reason this site reports a range of answers rather than one.