After an assistant accidentally erases the reminder about the Mayor's dinner party on the white-board calendar, she re-writes it back up...as she is leaving and someone else is entering the room, you can see she re-wrote it properly on Tuesday...but later in the movie it now appears on Wednesday.
** Mild spoiler ahead **
Whenever I see a continuity error like this in a David Mamet film, I always have to consider whether he put it in there deliberately (because he has done so in at least one other film). The confusion surrounding the calendar date is significant because it causes the movie people to embarrass themselves later by missing the dinner with the mayor and his wife as a result of the goof. I think the discontinuity could have been an intentional effort by Mamet to make the audience as confused as the characters. I sure hope he has commentary on the DVD when it comes out; maybe he'll explain this.