Chicago Cubs
The 1989 film Back to the Future Part II depicts the Chicago Cubs defeating a baseball team from Miami in the 2015 World Series, ending the longest championship drought in all four of the major North American professional sports leagues. In 2015, the Miami Marlins failed to make the playoffs and were able to make it to the 2015 National League Wild Card round and move on to the 2015 National League Championship Series by October 21, 2015, the date where protagonist Marty McFly traveled to the future in the film. However, it was on October 21 that the Cubs were swept by the New York Mets in the NLCS.

Q: What day were the Cubs swept by the New York Mets in the NLCS?
October 21