It is a term used in politics since the 70's and I hate to quote Wiki but also hate to type things that I thought everyone new...but here goes..

"In American political jargon, an October surprise is a news event with the potential to influence the outcome of an election, particularly one for the U.S. presidency. The reference to the month of October is because the Tuesday after the first Monday in November is the date for national elections (as well as many state and local elections), and therefore events that take place in late October have greater potential to influence the decisions of prospective voters.

The term came into use shortly after the 1972 presidential election between Republican incumbent Richard Nixon and Democrat George McGovern, when the United States was in the fourth year of negotiations to end the very long and domestically divisive Vietnam War. Twelve days before the election day of November 7, on October 26, 1972, the United States' chief negotiator, the presidential National Security Advisor Henry Kissinger, appeared at a press conference held at the White House and announced, "We believe that peace is at hand".[1] Nixon, despite having vowed to end the unpopular war during his presidential election campaign four years earlier, had failed to either cease hostilities or gradually bring about an end to the war. Nixon was nevertheless already widely considered to be assured of an easy reelection victory against McGovern, but Kissinger's "peace is at hand" declaration may have increased Nixon's already high standing with the electorate. In the event, Nixon outpolled McGovern in every state except Massachusetts and achieved a 20 point lead in the nationwide popular vote. The fighting ended in 1973, but soldiers remained in Vietnam until 1975.

Since that election, the term "October surprise" has been used preemptively during campaign season by partisans of one side to discredit late-campaign news by the other side."

October surprise - Wikipedia, the free encyclopedia

It is a term used in both parties and both will try and "save" something for that period that might swing an election.