Is "Οριστικοποιήθηκε" the right word to use if you want to say "The meeting on May 26th has been set" after some uncertainty because of several issues that had to be resolved before knowing whether the meeting can be held?
You could also say «Η ημερομηνία της συνάντησης οριστικοποιήθηκε» (with no mention of the date), or «Αποφασίστηκε οριστικά ότι η συνάντηση θα γίνει στις...».