In Britain and the (then-future) USA, September 1752 had three Thursdays - Wednesday, September 2 (Julian) was followed by Thursday, September 14 (Gregorian), a

In Britain and the (then-future) USA, September 1752 had three Thursdays - Wednesday, September 2 (Julian) was followed by Thursday, September 14 (Gregorian), and then there was Thursday the 21st and 28th. That month had only two Sundays and two Mondays. Other countries that made the Julian-Gregorian transition at other times will have similarly short months somewhere else in their history. There might be an instance of six of the same weekday in a month, if some area repeated the right day while moving from the Asian side of the international date line to the American side. Alaska's the only example I can find, but they also switched from Julian to Gregorian at the same time, so they had Friday, October 6, 1867 (Julian) followed by Friday, October 18, 1867 (Gregorian).