An October surprise is usually defined as the well-known (and more often left-wing) tactic of manufacturing or unloading a news story right before voting to surprise a rival without allowing them time sufficiently to respond or recover.
Think of the last-minute bombshell disclosure, five days before the 2000 election, that candidate George W. Bush had been cited for drunk driving over a quarter-century earlier. That surprise may have cost Bush the popular vote that year.
Sometimes, an incumbent can use his powers of office to warp the election. President Joe Biden benefited before the 2022 midterm elections when leftist activists leaked the impending Supreme Court repeal of Roe v. Wade.
Closer to the actual voting, Biden sought to cancel hundreds of billions of dollars of student debt owed to the federal government. He also began draining the strategic petroleum reserve to lower gas prices (as he is doing again this election year as well). No wonder the predicted Republican midterm red wave ended up a tiny ripple.
More often, October surprises are more ad hominem and unleashed on a rival candidate’s supposedly previously undisclosed failings.