Monster Day isn't a sale or a promotional thing. Monster Day is a holiday I made up that (some) other people actually celebrate, and you should, too!
Monster Day is February 16th. To celebrate Monster Day, go out and get markdown Valentine's Day candy on the 15th, or tasty vegetables, if you are so inclined. Then, on the 16th, you eat your snacks, watch monster movies and listen to monster songs, and draw monsters. On Monster Day, I'll make a post with some links to movies and whatnot you can watch, and invite you to link to your monster art on that thread, to share with everyone. It'll be great! :D