Introduction into the academic calendar.
There is a difference between the semester and the time where the courses are in session. A semester is always exactly half a year. The summer semester starts at 1st of April 1st and ends on 30th of September. The winter semester starts at 1st of October and ends on 31st of March. In a summer semester the courses are starting somewhere in the middle of April and they are ending in the middle of July. In a winter semester the same applies from the middle of October to the middle of February with a Christmas break at the end of December. Please find actual data at the academic calendar. Nobody knows why, but in Germany a summer semester lasts only 14 weeks and a winter semester lasts 16 weeks.
Some courses are provided outside of the periods named above. Please see the course catalog for details. Research projects are provided all-the-year.