When someone does something that’s unusual and nicer than normal, you may ask:
What’s the special occasion?
For example, you can ask this if:
someone is dressed better than normal
a room is decorated as if for a party
a friend is being unusually generous
The expression “What’s the special occasion?” is very well-known among English speakers. For this reason, people sometimes simply ask:
What’s the occasion?
...which means the same thing.