Fuck, is Hammer Day every month?
Every month. It's a superb culture founded by the first Hammer person. The history is on the nicolopedia.
ZUN mentioned commentary playthrough videos. I was searching for ones that don't require Japanese... Fortunately, the two most popular ones don't.
Super Mario 64, with additional rules. Activate the eternal chasing life-up mushroom. Get all 8 coins and the star without getting caught by the mushroom. Although knowing Japanese would increase the laughter level because of flag retrievals and skits, I think it's still fairly enjoyable. Most popular on nicovideo. Done by Sakamoto Ryouma (player) and Saigou Takamori (friend). (Names come from edo era samurai.)
Super Mario 64, THE GREEN DEVIL part 5:
http://www.nicovideo.jp/watch/sm6188097part 6:
http://www.nicovideo.jp/watch/sm7552074Super Mario Brothers, played 4.2x normal speed. The editing is phenomenal. You don't need to know Japanese, because he's doesn't speak Japanese most of the time... Second most popular on nicovideo. Done by P. (Yes, he says Pikachu.)
Super Mario Bros., high-speed play part 3:
http://www.nicovideo.jp/watch/sm6478737Super Mario Bros. 3, high-speed play part 1:
http://www.nicovideo.jp/watch/sm5157574I Wanna Be the Guy, an American hellishly hard side-scroll action game by Kayin, played on a keyboard with tremendous skill. If you don't know the game, it'll make you laugh.
I Wanna Be the Guy, The Ultimate Hell Game:
http://www.nicovideo.jp/watch/sm5662543Aooni, a Japanese mystery horror game, played by 3 people, a sadistic onlooker and a screaming player. The screaming brought this first video 500,000 watches in the first month. Headphones with max volume please.
Aooni, I made my friend who's known for his screaming commentate it:
http://www.nicovideo.jp/watch/sm6979644I can introduce more if you're interested. You might get what ZUN is talking about better that way.