Depending on the university, the academic year starts sometime from late August to mid-September. There are two ways in which the academic year can be organised in US universities:
Semesters
Most universities use the semester system where there are two major periods of study in each academic year – Fall and Spring. There are 16 or 17 weeks of study in each semester. There may also be a shorter optional period of study in the summer.
Quarters
Some universities use the quarter system where there are four 10 week study periods (or quarters) during the year. You are usually expected to attend three quarters, but may attend all four to graduate sooner. These are sometimes also called “trimesters.”