I can't saw what the actual largest post count day is but I remember back a while where some of us had 200-300 posts in a single day. There was a string going for a while where we had SEVERAL 1000 plus post days.

Oh and just for trivia's sake, the 750,000 post goal we are always talking about was given to us by Sumo275, aka Jeff (NOT Jeff as in Rainwalker).

We had a sorta unofficial contest to try and get to 500,000 posts by blade 2011, and we made it and then some. So we got to talking about a new goal. And now you know.