dirty tricks Thursday
n. The Thursday before an election when candidates often accuse opponents of scandalous behavior knowing that the charges will bring headlines while also allowing the opponents little time to refute the charges.
Forget Halloween. Today can be one of the scariest days of the year. We call it Dirty Tricks Thursday. With just a few days before the election, ghouls and goblins creep out from their lairs with warnings of doom.
—“Lyons: Scary election story? What took you so long?,” The Northwest Herald (Crystal Lake, Illinois), November 01, 2012
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 05, 2003
2003 (earliest)
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 03, 2003
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