The idea is that anybody who wears a sundress will get happy hour all day -- both dudes and ladies. You can bring your own, but they'll also have racks and racks of them for you to wear in all sizes, from the nice to the ridiculous. There's a lot of camaraderie, and it's a pretty funny sight to see that bartender who only seems to wear black t-shirts in a flowery little number. Here's Wonderland's page about the event.
Plus, happy hour deals. The atmosphere is great and it's one of those times when you'll probably see some neighbors or pals walking by or having a drink too.
The Wonderland Ballroom is at 11th and Kenyon.