Chandra

your festivals move because the sun and moon refuse to divide evenly.

drag through the years on a planner: Christmas stays put, Eid walks back through every season, Diwali hops back when a month is added

Twelve moon months make 354.37 days, about 11 short of the sun's 365.24, so Eid comes 11 days earlier each year and goes round every season in about 33 years. Christmas follows only the sun and stays put. Diwali follows the moon too, but its calendar adds a month (adhik maas) whenever it falls a month behind, so it hops back and stays in autumn. A model of average rates, not a real calendar.

Concepts: moon, chronology.

An interactive you play with one thumb, on Wundr.