General:
* Theme Day
* Costume Day
* Dress-Up Day
* Spirit Day
Specific Themes:
* Decade Day (e.g., 80s Day, 90s Day)
* Decade Dress-Up Day
* Character Day
* Superhero Day
* Hawaiian Day
* Formal Day
* Casual Day
* Pajama Day
You can also use phrases like:
* Day to Dress Up
* Day to Express Your Style
The best word or phrase to use will depend on the specific context and the theme of the day.