The day of arrival and departure are part of the FUBiS program. In Term I arrival and departure days vary. The specific dates can be found here: http://www.fubis.org/2_prog/termine/index.html. For Term II and III, it usually is the first and the last Saturday of the respective program. You should arrive in Berlin and move in to your accommodation on the day of arrival. Please note that the day of arrival is the earliest you can move in to your accommodation. Here you can find the arrival and departure dates for your respective term.
The FUBiS orientation meeting is taking place on the day after the official arrival day of the respective terms. Here you will receive important information about the program and your courses. Therefore, you should make sure that you arrive in Berlin on the official day of arrival (usually the day before the orientation) at the latest.