Can you help me brainstorm some events that are supposed to happen on a predictable regular cycle, several years apart? In addition to:
* Leap year happens every 4 years. (US Presidential elections are in leap years.) * You're supposed to get a tetanus booster shot every 10 years. * Periodical cicadas emerge every 17 years. * Halley's comet returns every 74 to 79 years.
This is for a project that I'm working on. What other cycles like this can you think of?