There are no laws concerning the length of the academic year. The only rule is that there have to be 60 credit points awarded to each student. The 60 credit points equals about 1680 hours of study.

There are two examples of how universities divide their year: the 2 semesters of 20 weeks academic year , which starts in September or February and the 4 semesters of 10 weeks academic year , which starts in September, November, February and April. It depends on the university you are going to on how many tests you get and how your academic year looks like. 

You can only start a study at the beginning of September or February. A study in the Netherlands contains 40 weeks of involvement and presence and 12 weeks of holidays which are spread over the entire year. Of course, there are exceptions if a student falls behind during his study. Then, it is possible to study during the summer.