Most universities in the US (90% according to [this website](http://study.com/academy/lesson/semester-vs-quarter-schedule-pros-and-cons.html) according to), are on *the semester system* in which there are two academic terms, fall semester (sept-dec) and spring semester (jan-may). In this semester system, exams happen at the end of each semester, not in a separate time period.

Some universities have a *quarter system*, where each quarter is identified with a season and students typically have the summer quarter off. Still other universities have a *trimester system* which "divides the academic year into three terms of 14 to 16 weeks each, with attendance required in two trimesters per year, to total 28 to 32 weeks of instruction" ([wikipedia](https://en.wikipedia.org/wiki/Academic_term)).