Turns out it's for the same reason we're having a Tuesday game this year: bad weather. The New York Giants and Boston Yanks were set to open their 1946 regular seasons on Monday, Sept. 30 at Braves Field in Boston, but a torrential downpour forced the teams to move the game to Tuesday.
Mental Floss quotes that day's New York Times: “Waiting until early afternoon for a change in conditions, Owner Ted Collins of the Yanks and Stout Steve Owen, the Giants’ coach, agreed to put the contest over until tomorrow night.” The game was moved to Tuesday, which was not unheard of back in the days when stadiums were shared by a number of teams and travel was unreliable. A total of 21 Tuesday games had been played before that night in 1946. (Also interesting to note that the team's opener was scheduled for a Monday, not Sunday. Looks like Monday Night Football predates Roone Arledge and Howard Cosell.)
The teams took the field in front of a small crowd, chilled by what the Times called a "sudden turn of the weather." The gathered 16,000 didn't see much of a game. According to the game story "the high-powered Giants team beat Ted Collins' squad -- 17-0, in a game that had some highlights, but one that for the most part was dull as a result of its one-sidedness." (That's some frank reporting right there.)
- Boston Yanks