As far as i know
on the assesment days you have to make an appointment and if they invite you, you can go. On the open days everybody can go and it goes like this:
-Informative video and giving CV's to the recruiters, that's the first selection
-1 Group work and also they measure how tall you are, that's the second selection
-2 Group work and third selection
Then the people that made it have to make english tests, personality tests, etc. And you have to decide the hour wich you are going to your final interview that is going to be maybe the next day or next week it depends.
Then in 6 weeks you will receive the golden call (gc) or the rejection email
Hope this helps you!!