Introductory events are usually held in the week before lectures start. Use this week to familiarize yourself with university life and the campus. Visit - perhaps together with other first semester students - the important learning and information venues, such as the Institute Library, the University Library, the Center of Data Processing, the lecture theaters that are relevant for you, etc.
Preliminary courses, which can take place in the weeks before lectures begin, are available for Mathematics (for students of natural sciences) and for Slavic Philology languages.
Alongside introductory events that are only relevant for a specific subject, interdisciplinary events are also available, e.g. relating to subjects such as obtaining state financial support (BAföG), studying abroad, teacher training courses, etc.
All dates, as well as a wealth of other useful information, are available in the brochure "Beginning your studies", which will be sent to you by post together with your matriculation certificate.
The brochure can also be downloaded here.