Best day because of 2 football games? No way. More to the sports world than the NFL. Has to be some other things going on.
I think McNulty has the general idea, combining some EPL and some Aussie Open in there as well. But you can't really always count on those things.
But having said that, not really sure I have an answer. I'd probably go either Opening Day or first weekend of NCAA tournament, but that's just because I'm more a fan of those sports, and also it's based on more than just 2 games.
What's wrong with your reasoning being that you are a fan of the sport(s)? That's why I'm choosing what I choose. If I'm a horse racing fan, I'm probably saying the Kentucky Derby. If I'm an auto car racing fan, I'm saying either the Daytona 500 or Indy 500. There's no "right answer" here.
I also think that the amount of games can matter as much or as little as you want. The two games today last pretty much the whole day, when you include pregame/postgame stuff. Now you bypass all that, but still it goes from about 3 to 10.