Unlike Christmas, Hanukkah does not fall on the same dates each year because Jewish holidays are based on a lunisolar calendar. Unlike the traditional Gregorian calendar, which is based on the earth's movement around the sun, a lunisolar calendar is regulated by both the sun and moon. Hanukkah always starts on the 25th of Kislev — the month on the Hebrew calendar that generally coincides with November or December. A year on the Hebrew calendar varies from 353 to 385 days, so the timing of the Festival of Lights is always a bit of a guessing game.