I will be a senior in high school next school year.
Sometime during the first semester of your senior year. Deadlines vary from college to college. For regular decision, the deadline is usually early January of your senior year. I think early decision and early action are pushed forward to around October of your senior year. The best thing to do is research the college or colleges that you’re interested in and find out when their dates are.
the beginning of your senior year. You could start preparing your personal statement if you plan on going to a state or UC. The most common time is during the beginning of your senior year, other wise you're screwed.
During they're registration dates.
when school starts or summer?