Really? You can't use τι for "what" in this sentence?
Ποιος/ποια/ποιο are used for both 'who' and 'which one'. Τι is used to literally ask 'What is this?' Saying "Τι είναι η επίσημη ημερομηνία;" means 'What does η επίσημη ημερομηνία mean?' So, no, you can't use τι here. :)
Καλώ explanation! :)