I don't know if this is feasible or not, but the ability to spawn mobs specifically during Bloodmoons would be sweet.

https://www.curseforge.com/minecraft/mc-mods/bloodmoon

Yes I could really use this too. I love both of these mods.
Also, a moon phase or an "every x days" condition would be helpful

This is something I've wanted to do in my pack for a very long time. By default, blood moon is kind of boring in that it just applies a flat increase to mob spawn caps/rates. It'd be nice to change that so that certain mobs only spawn during a blood moon or something along those lines- so it's more interesting and less mob spawn spammy

Pardon the bump, but it would be absolutely amazing if this could be extended to Enhanced Celestials support: https://www.curseforge.com/minecraft/mc-mods/enhanced-celestials

Problem is that the mod has no API which makes it practically impossible to support. I tried asking the mod creator but he doesn't seem to have time for this

Do you know if that's still the case? They seem to have overhauled their systems recently, perhaps it's more open to third party tools now.
If not, I'll go ahead and open an issue requesting compatibility on their end.