Calendar
Hanukkah always begins on the 25th of Kislev in the Hebrew calendar — but on the Gregorian calendar, that can land anywhere from late November to early January.
Unlike Diwali or Chinese New Year, Hanukkah's Gregorian date swings across more than five weeks from year to year — from late November all the way to January 1. That's because the Hebrew calendar's leap-month system (an extra month added in 7 of every 19 years, a cycle known as the Metonic cycle) is tuned to keep Passover in spring, not to keep Hanukkah at a stable point in the Gregorian winter. Hanukkah just falls wherever that adjustment happens to land it.
Every so often, Hanukkah's first night coincides with December 24th, as it does in 2027 — an alignment that happened previously in 2005, 1959, and 1921, and won't happen again until 2035. It's a direct, visible marker of how far out of sync the Hebrew and Gregorian calendars can drift within their shared 19-year correction cycle.
Related
Another movable holiday, on the Hindu lunisolar calendar.
For duration math once you have a fixed date to work from.
A live countdown to Hanukkah, Christmas, or any custom date.