pitchi,
open days are exactly that: open to all...
the qatar assessment takes place over three days:
1st day: open day
You should dress very smart (as you have to make an impression amongst a crowd)
At this day they give you information about the airline and life in doha.
You then line up to hand in your cv and photos, and if you are lucky, you will be called to attend the assessment the following day...
Day 2: the assessment.
The selected assessment candidates return (Dressed appropriately)
You write a short essay, do a reach test and some group discussions.
They eliminate people twice during this day, and by the end of it, those who remain will choose a timeslot for their final interview.
Day 3: Final interview, one on one about you, your experiences and how you deal with things.
The recruiters are great, very open and if you have any questions they are pretty upfront about things, they warn you that the airline is strict, but that they have a 5 star reputation to maintain.
anyway, I hope this helps you.
Good luck with everything!