"Those 2 days are the day before and the day after the MLB Allstar game. They are travel days for the players participating and therefore no games can be played."

That's the old answer. Since the NHL began taking a hiatus for the winter Olympics, the more complicated answer depends on whether or not the hiatus surrounds one of both days of the NBA All-Star game. In some winter Olympics years, the NBA All-Star game occurs entirely within the NHL hiatus, so on the days before and after the NBA All-Star game, no major North American professional sports - NBA, NHL, MLB, NFL - are played. This year (2010), the NBA All-Star game was played on Sunday, Feb. 14. NHL games were played on the Saturday before the NBA All-Star game, but not on the Monday following.

So, the new answer is, there might be more than two days, and it all depends on where the NBA All-Star game occurs relative to the NHL hiatus. In some years, there may be as many as four days in which none of the major North American pro sports are played.

And there is an even newer answer as of 10/5/10. On the day before the MLB playoffs starts, there are no professional sports regular season games played(pre-season does not count) as the NFL does not play, the NBA & NHL are in pre-season and there are obviously no MLB games until the next day. This answer is subject to change even further in the future as a MLB one game division playoff game could be played on the Tuesday before the MLB playoffs starts because as it is usually slated to be played on Monday, it could be weather postponed and delayed until that Tuesday.