Today is a day to celebrate your casual pride. You can be introspective (outwardly) about the pride you have in your own casualness or the casualness of those you love and/or support. It can also be a day to commemorate anything at all of which you are just casually proud.
This day of days can be (but doesn't have to be) consecrated by the wearing of the jeans (or sweat pants if you're super dedicated to the cause. A Forever Lazy would make you some kind of Casual Pride Santa Clause). Throughout the day be sure to do pretty much everything casually. Show up to work casually late. Have casual sex (if you're into that). Casually dress someone down for being anti-casual. Casually bump into that hot guy at work and give him a thumbs up while casually saying, "Fuck me Thursday, hotness." When waiting in line, always keep your weight sitting casually over one hip or the other while having casual sexting sessions with strangers who are checked in at the same 5 Guys as you. Just whatever you do, do it casually and proudly.
Here's what: if you didn't have time to prepare for the holiday today, don't worry. We're pretty casual about the date. The holiday falls near the end of June, beginning of July, but we're not really married to that date. You can celebrate it in November if you want. Whatever. We're casual about it.
And if you work in a place where casual just isn't done (I'm looking at you Jamie), then just take the day the fuck off. Casually call in sick.