I really can't imagine. There's already the duffel coat for seasonal weather, and already the Sweet Duffy Cafe uniforms for Valentine's through White Day. The only 1/21 holidays I could find with a quick search were both in the United States:
- National Hug Day - This seems appropriately "Duffy," but what's the costume?)
- Squirrel Appreciation Day - While I think Duffy and Shellie in squirrel costumes would be super-cute - even though Rilakkuma and Korilakkuma just did this last autumn (as you can see in this Yahoo store) - I kinda doubt that's really it.
So I'm just gonna take a wiiild guess and say it's some new variation on the white sailor uniform.
Okay, your turn! Play along? Pretty please?^^