How do you say Happy Friday in Japanese?

It's as odd in Japanese as it sounds in English. General expression for congratulating in Japanese is omedetou goazimasu, or omedetou alone. Japanese for Friday is kin'youbi. Slapping them together would result in:

"Kin'youbi omedetou gozaimasu" in which, in casual form 'gozaimasu' is not mentioned.