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?

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