Actually, it depends on the jurisdiction. Your state supreme court's website should have the deadlines. The administrators at your school should also be able to answer your questions.
In my state, law students must register as a "Candidate for Admission" by November 15 of their second year. This involves a very intense background check and an in-person interview with a member of the admissions committee. Then, 3Ls must register by April 1 if they wish to take July bar exam.