What happens during a competition day?

Tomorrow must be a candidate day for a partial qualification!

I wanted to ask how long does it take? And what do you do there? Do I have to register first or do I have to stay there the whole day tomorrow?

Depending on the operation:

self-presentation
employment test
Solving a problem with other candidates in the team
Job Interview
Company tour

As a duration you can plan so 3-4 hours. But this is very dependent on what is required of you (above gennantes).

Self-presentation would be announced or?

[Posted posted]

Usually with the keyword "Assessment Center", but in the normal case, you will be notified what to expect.