Some politicos dub the Thursday before a big election "Dirty Tricks Thursday." That's the best day for an opponent to unload his bag of filth against another candidate, getting maximum headlines, while giving his stunned opponent no time to credibly investigate or respond to the charges.

It creates a Black Friday, where the candidate spends a precious business day right before the election desperately investigating the accusations, before facing a weekend in which reporters only care about further accusations that invariably spill out of the woodwork.
—Jill Stewart, "Covering for Gray," The Daily News of Los Angeles, October 5, 2003

I think the worst thing you can do is attack him on what's called dirty tricks Thursday, the Thursday before a major election, which is the day that all of the opposition camp's attacks are made. Generally newspapers don't join in the opposition camp's attacks. It's called dirty Thursday because it's too late for a Schwarzenegger or whoever a newspaper attacks to come up with answers.
—Bill O'Reilly, "Schwarzenegger Faces Accusations," The O'Reilly Factor, October 3, 2003