Last weekend, the Indianapolis Colts of the National Football League faced off against the Minnesota Vikings. For the Colts, it was a meaningless game, for the most part; with only four wins (and one tie) through their first 13 games, they were all but eliminated from the playoffs. The Vikings -- 10-3 going into the matchup -- were heavy favorites. The Colts shocked everyone by taking a 33-0 lead going into the half, which, if you're unfamiliar with football, is a very sizable lead -- no team in NFL history had ever come back from such a deficit. But there's a first time for everything I guess, and the Vikings ended up winning, 39-36 in overtime.

But the strangest part of the game? Okay, it was the comeback, fine. The second-strangest, though, is that this was the first Saturday game this season for the NFL.

Why? Because for most of the fall, it's effectively illegal for the NFL to play games on Saturdays.

If you're a fan of the NFL, you already know that Sunday is typically game day. There are multiple games starting at 1 PM ET, then another set at around 4:30 ET, and finally the "Sunday Night Football"-branded game that evening. The NFL has also played a Monday Night Football game weekly since 1970 and a Thursday Night Football game weekly since 2006. Monday through Wednesday are recuperation days -- football is a brutally physical sport, and you simply can't play every day.

Friday nights and Saturdays, though, typically don't have games. And that doesn't make a lot of sense on its face. If teams can be healthy enough to play on Thursday, they can certainly play on Friday or Saturday. And both Friday night and Saturday are, basically, the weekend, which means there's plenty of time for fans to kick back and watch the game. 

And if you ask the NFL, they'd really like to play games on those days. But by law, they can't. The problem started in 1961 when the NFL scored its first league-wide television deal, giving exclusive rights to CBS. Before that year, individual teams negotiated their own television contracts, which led to a lot of competition for the games among broadcasters. As Sports Business Journal notes, the U.S. Department of Justice didn't like the NFL/CBS deal, believing it was anti-competitive. So the DOJ sued the league, arguing that the league-wide TV deal violated antitrust law -- and the Justice Department won.

That spring, the NFL appealed to Congress, asking lawmakers to change the law. Congress agreed, passing the Sports Broadcasting Act of 1961 just a few months later. The law provided an exception to the antitrust law, allowing the NFL's deal with CBS to go forward.  The exception, though, had an exception to itself. High school and college football games are typically held on Friday nights and Saturdays, so, to protect these amateur leagues from the appeal of pro ball, Congress created a carve-out in the Sports Broadcasting Act itself. As Slate explains, there's a blackout period for NFL games: from mid-September through mid-December, NFL games cannot be broadcast on Fridays (after 6 PM) or on Saturdays "within seventy-five miles of game site of any intercollegiate or interscholastic football contest scheduled to be played on such a date." 

That's a pretty big radius, so the NFL just avoids Friday nights and Saturdays altogether during those weeks. Sometimes, that's unavoidable -- for example, in 2004, a hurricane required the league to move an early-season game from Sunday to Saturday; to avoid running afoul of the law, the game was televised only in local markets live on that Saturday, with the rest of the country getting a replay the next day. But those exceptions are few and far between.

The blackout period ends after the second Saturday in December, and from then until the end of the NFL season, there is professional football on any given Saturday. That extra day of football is great news for fans -- unless you were rooting for the Colts last week.