Bonjour dino330!
Bienvenue chez pprune!
The Open Days are for everyone that is interested in becoming cabin crew with Emirates.
They will make you sit an English exam, and then conduct some brief interviews. There are usually a large number of people there, but you don't need an invitation.
Be positive, be prepared, but don't get your hopes up. I have been twice and haven't gotten through...
Write your online application number at the top of your resume, as they like to know your details are on file.
Bon chance!