How do I prevent Calendar from draining my iPhone battery?
One of your Calendar\Mail\Contacts accounts is in an endless loop. Maybe a corrupted event or something of that sort.

What iOS are you using? I remember something of that sort in iOS 8, no idea if this was fixed yet.

First I would suggest removing one account at a time and check after a day of usage if Calendar is still going crazy. If it does, move on to the next account. Once you figured out which account is the problematic one, add it again and configure it as new. This should fix it.