It would be cool to have a handful of rituals that need a certain phase of the moon to be performed (e.g. some kind of dark ritual requires a new moon, or the light of a full moon). They could give some unique bonuses to provide incentive to keep an eye on the moon phase.

As an extension to this, perhaps the weather could also have an effect on some rituals.

It's not a bad idea, but it would need an effect worthy of a tricky prerequisite. I'll keep it in mind.