You must know Hebrew, and math, fairly well to get this one:
How is the number 40,320 associated with a Jewish holiday?
Hint: Reduce to prime factors.
This isn’t really a riddle, but my son said that Isru Chag Sukkot (the day after Sukkot, and the last day of vacation) is also known as Yom Hacharisha Ha-leumit (national ploughing/grubbing day). The kids use it to do all the homework they neglected over vacation.