TYB
In American political jargon, an October surprise is a news event deliberately created or timed to influence the outcome of an election, particularly one for the U.S. presidency, or sometimes an event occurring spontaneously that has the same effect. Because the date for national elections (as well as many state and local elections) is in early November, events that take place in October have greater potential to influence the decisions of prospective voters. Thus these relatively last-minute news stories could either completely change the entire course of an election or strongly reinforce the inevitable.[1]
The term "October surprise" was coined by William Casey when he served as campaign manager of Ronald Reagan's 1980 presidential campaign.[2] However, there were October election-upending events that predated the coining of the term.
https://en.wikipedia.org/wiki/October_surprise