I think firemen work 24 hours on, 72 hours off. I think Kelly days are extra, random days (or time) off to allow for there being no overtime pay. For example, if you work two 24 hour shifts, you would have worked 48 hours in a week (overtime), but if you are given 16 hours off over a two week period, you would still have only worked 80 hours. (Don't know if that made any sense.)
My best friend from nursing school explained it to me this way. She is married to a fireman.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?