Werewolf-type creature transformations - what could trigger them?

In my current book, I've created my own version of a werewolf. They transform into decidedly different types of creatures, and I've invented my own abilities and limitations for them; so they're werewolves except that everything is changed a little, if that makes sense.

The one change I can't figure out, though, is when to make them transform. I need them to transform at regular intervals, but the full moon is already taken, obviously - so, is there anything else you can think of that happens (or could happen, in a fantasy world) on a regular basis, which could trigger my monsters' transformations? I originally thought I'd use a set number of days (everything in this world is based around the number five, so I chose every twenty-five days because that seemed reasonable), but I don't like it. I like the sort of concrete, cosmic cause that the full moon provides, and I wish that I could think of something more like that - a set event of some sort. Any ideas?

 

Thanks for the suggestions! And sorry, I guess I should have provided a little more information! These lycans, I suppose you could call them, are somewhat unique in that their beast side slowly takes over their minds so that, after a while, even in human form they are completely evil. They end up siding with the arch villain of the story who frees them from the prison where these creatures are usually kept for safety. One reason I'm being careful about how often they can transform is because I can't overpower the villain *too* much! But also, and more importantly, I need it to be a set time because one of my main characters develops this condition when he is very young; but unlike all the others, he manages to keep the beast side from taking over his mind (except when he actually transforms). The king discovers him and, because he needs at least one of these creatures on his side if at all possible, and because he can see that this unusual boy is still good at heart, he decides to nurture him and raise him lovingly so that he'll end up being on the king's side. Part of this includes providing a magical object that makes the little boy appear human, and allowing him to attend the royal academy, make friends, be a normal person, etc.

So, in order to do that, that means that there's got to be some way that the king can know for sure when this kid is going to explode into a rage monster, so that he can be sent somewhere safe where he won't hurt anyone, until he returns to normal.

I am thinking that the evil beasts will discover how to transform at will in the future (with the villain's help), but for now, no one can know that's possible, or I feel like the king just couldn't risk exposing all these kids to this little boy because he would be putting their lives in constant danger.

 

Yes, exactly! I know it sounds funny when you put it like that; but imagine that this is a medical condition - a biological response. The way I've got them worked out, transforming at will would be something akin to a person with epilepsy having a seizure at will. They may eventually figure out how to trigger one - perhaps the oldest of them harbors this secret knowledge but has hidden it from the humans, and will reveal it after the villain frees them, or maybe they will simply discover it. It has never been studied, and those affected have always ended up captured and magically contained, if not killed (there's an elite force of the military dedicated to doing so), and thus living isolated from any other beasts. They haven't had much chance to learn about themselves.

To be clear, my sort-of lycanthropy in this world is more of a sickness that turns people into powerful but evil monsters, rather than a superpower that gives people the ability to be awesome.