Bukkit

This plugin produces an event: MoonPhaseChangedEvent. It is fired each time the moon phase changes. It acts just like any other bukkit event.

I don't think it needs too much documentation. You can take a look at what it can do here:
https://github.com/MrZoraman/MoonPhaseReactor/blob/master/src/com/lagopusempire/moonphasereactor/events/MoonPhaseEvent.java

The other important file to take a look at is the MoonPhase enum itself:
https://github.com/MrZoraman/MoonPhaseReactor/blob/master/src/com/lagopusempire/moonphasereactor/MoonPhase.java