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.