1) wearing casual clothing (jeans, t-shirt) on a Friday.
2) a practice common with high schoolers where you don't wear underwear under your pants on Friday. It's really funny and really is only funny to the people that do it. But none the less, it's not as weird as you'd think.
Person: "Emily, are you doing Casual Friday today... because I noticed you're not wearing a skirt like usual."
Me: *laughs hysterically* do you even NEED to ask?
"What are you wearing to school for 'Casual Friday'"?
"That's for me to know and you to find out."
(speaker almost gets pantsed.)
"Okay, okay. I'm wearing my favorite orange jockstrap. How about you?"