First of all stella this is a forum for pilots, here is the link for cabin crew.
Cabin Crew Wannabes - PPRuNe Forums
As for your question, sometimes your online application invite is for assessment in which case you do not have to attend an open day. But since you say you applied for open day you will need to be called back in order to go to assessment day.
you also do not need an invite. In my opinion it is best not to apply, that way if you do not make it to the final interview you can attend another open day within 6 months as they will have no proof that you ever attended one and were in a waiting period. Good luck,I hear Greece always has hundreds of applicants.