Random things has a speciall event that has a configurable chance of happening called the "Bloodmoon", similar to Terraria.

It would be really nice if there was a tag that could use this as a condition for mob spawning. It would allow mod pack makers to assign specific mobs to spawn duing a bloodmoon - or in my case, make it so that mobs only spawn on the surface during a bloodmoon, but do so at 3x the rate.