A semester always contains a lecture period (“Vorlesungszeit”), in which tutorials, seminars and lectures take place, as well as a lecture-free period (“vorlesungsfreie Zeit”). For example, whilst the Summer Semester 2016 runs in total from April 1 to September 30, courses only take place between April 18 and July 23. Some teaching does also take place in the lecture-free period, albeit significantly less. Intensive language courses, such as “Deutsch als Fremdsprache”, often run during this period.
The opening hours of certain libraries, canteens and other institutions differ during the lecture-free period. It is also usual for professors to offer only individual appointments for consultation hours.
Further information and dates can be found here.