Normally the winter semester starts in October and the summer semester starts in April. For october normally the deadlines are till July 15th and for Summer Semester the deadlines are till January 15th.
But students should try to submit their applications as early as March 15th for the winter semester and November 15th for the Summer semester.
Note: However it should be noted that because of good amount of time required for the visa application the students should submit their university applications earlier and also the competitive universities have earlier deadlines in some cases almost one year ahead of the the start of the program. And it is also the same for DAAD scholarship.