Most Home Runs in a Doubleheader

There has never been a player who has hit five home runs in one Major League game (it has been done a number of times in the minor leagues). There have, however, been two players who have hit five home runs in one day. Thanks to doubleheaders, Stan Musial and Nate Colbert were able to achieve that feat:

On May 2, 1954, St. Louis Cardinals great Stan Musial hit five home runs in a doubleheader against the New York Giants. Though he went 6-8 with five home runs for the day, he only drove in 8 runs.

On August 1, 1972, San Diego Padre Nate Colbert equaled Musial's feat and did him one better, hitting five home runs and driving in 13 in a double header against the Atlanta Braves. Interestingly, Colbert grew up in St. Louis and, in post-game interview with Tom Emory of the Associated Press, claims to have personally witnessed his boyhood idol's historic feat (correctly identifying both the venue and the day of the week):

I was there when Stan hit those five in old Busch Stadium. Hitting five in one day is unreal. I don't remember the exact day, but I know it was a Sunday. Stan was my first hero when I was growing up. I'd be pretty proud to have the kind of batting average he had.