Suggested Ideas thus far:
"Friday" Day - Partying! Partying! Yeah! Dress your avy based on the famous song by Rebecca Black (suggested by SheThoughtSheDid)
Song Day - Let the Music Play and dress your avy based on your favorite song or artist (suggested by Small Plushie)
I Be Poor/Flaunt Your Wealth Day - Whether you are poor or rich, let your avatar show it all off (suggested by antipolar)
Culture Day - Represent your heritage as best you can through your avi (suggested by lil4fingers)
Crossdressing Day - Don't be a drag, just be a queen. Dress your avy as the opposite gender (suggested by Innocently Yours)
Harry Potter / Twilight Day - Live your fantasy, dress your avy based on one of these popular books. (suggested by Death T-2)
Literature Day - Make the words come to life by dressing your avy based on one of your favorite books (inspired by Death T-2's suggestion)
Cute Day - Love lolita or think Dandy is dandy, dress your avy in whatever cute way you can think of (suggested by Genea)
Superhero/Villain Day - It's a bird, It's a plane, It's your avy dressed like your favorite super hero. Or maybe you prefer the darkside, dress your avy like your favorite villain. (cookies not included) (suggested by SheThoughtSheDid)
Elvish Day - Ando Eldarinwa a lasta quettanya, Fenda Casarinwa! Dress your avy like an elf. Man cenich? (suggested by animekittyninja-aka kiki)
Yu-Gi-Oh Day - Got to catch em all It's time to d-d-d-d-d-duel! Dress your avatar like your favorite character or even your favorite duel monster. Your Move! (suggested by animekittyninja-aka kiki)
Elemental Day - By your powers combined, I am Captain Planet. Dress your avy like your favorite element. Master all 4 elements. (suggested by animekittyninja-aka kiki)
Internet Meme Day - Been itching to see a cat play a piano or to set off some diet cokes and mentos? Dress your avy like your favorite internet meme. Do it or we'll sick Boxxy on you. (suggested by Flamethrower.x)
IRL Day - Instead of dressing up like your avy, dress your avy like you. IRL Day is all about being yourself (Be who you want to be!) (suggested by Flamethrower.x)
Rainbow Day - Taste the rainbow! Or just dress like one. Show off all the colors in the rainbow and dress up in your favorite rainbow items. (suggested by Duo-chan_in_wonderland)
Character Day - Have you dreamed of being Spiderman or Dr. Who? How about Harry Potter or Indiana Jones? Embrace your fantasies and dress up like your favorite character (TV Shows, Movies, Books, Comics, etc.) (suggested by Duo-chan_in_wonderland)
SD Plus Day - I love playing with dolls. Really, I do. Dress up your avy to include your favorite SD Plus doll (or your entire collection). (suggested by Duo-chan_in_wonderland)
Pajama Day - Sure we've had it before. But who doesn't like lounging around for an entire day in their pajamas. We can all hum along to The Lazy Song and sip hot cocoa. (suggested by Duo-chan_in_wonderland)
I Am Day - Have you ever wanted to be someone, ne, something else? Today's your day to live out that fantasy. You can be a cow on the moon! Be whatever you want to be (regardless of what your parents or law enforcement think of it). (suggested by ThatOddBlonde)
Disney Day Do you want to go to a whole new world? With dancing furniture, mermaids, and little blue monsters! Become a character, and relieve your favorite childhood disney movies. (suggested by ThatOddBlonde)