“What's the special occasion?”
Your husband brought you flowers. He doesn't usually do that except on your birthday or anniversary. You ask why he brought them.
What's the special occasion?
A “special occasion” simply means an important day or event. Special occasions can include birthdays, graduations, weddings, and more. They are usually happy events that we celebrate.
Use “special occasion” like this:
I only wear a suit for special occasions.
If your sister gives you a nice bottle of champagne for your birthday, you might say:
I’m saving this for a special occasion!
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.