My period come down a different day each month

If you are under 23-25 or overweight your periods are subject to change or be irregular. If there are any major stresses, or changes in your everyday habits it can also have this affect.

That's pretty normal. Even with birth control, my period usually starts on Thursday, but its not unusual for it to start on Wednesday or Friday. If yours is especially unpredictable, you may want to talk to your dr.