The NHL does not allow for games, practices, travel or any team activity to take place on Christmas Eve, Christmas Day or December 26. It's in the collective bargaining agreement and is the law of the land, and it looks as though the Philadelphia Flyers violated that rule this year.
Here's a look at the Flyers charter flight from Philadelphia to Nashville on December 27, via FlightAware:
And here's the relevant portion of the NHL's collective bargaining agreement:
December 24, Christmas Day, and December 26 shall be off-days for all purposes, including travel, and no Club may request a Player's consent to practice on such days for any reason, provided, however, if December 26 falls on a Saturday and the League has scheduled NHL Games on such date, December 23 may be substituted as an off-day for all purposes, including travel, instead of December 26.
So, OK .... sure, the Flyers may have violated the rule by traveling on December 26. But it's a stupid rule -- one that would have hurt the Flyers more had they not broken it.
According to Sportsnet, which broke this story, the reason they ban travel on the 26th is to keep every team on an even playing field. It's a competitive balance issue, essentially. But by banning travel on the 26th and scheduling games on the 27th, the NHL wrote into the rules a clear advantage to every home team playing on the 27th.
None of the other teams in the league seemed to have an issue traveling on the 27th, but just because the other teams in the league complied with the rule doesn't mean it's a good one.
NHL teams almost never travel on the same day as a game -- even in the event of back-to-back games, the team quickly packs up and leaves immediately after the first game so that they can arrive in the second city, sleep there, and wake up with a normal day in the same city they will play the second game. Hockey players are very routine-based during the regular season, and this is one way they stick to a routine.
Also, have you ever traveled? It's tiring, and yes ... that's even if you're on a chartered, catered flight. You wouldn't want to travel and then play an NHL game later that day either, even if ... yeah, obviously it's not the end of the world if you have to fly the same day you play a game.
By banning the Flyers from traveling on December 26, the NHL was forcing them to travel on a game day. If the reason this rule exists for competitive balance, then forcing teams to travel on game day is more of a competitive balance issue than letting teams fly the night prior. It gave more of an advantage to Nashville than anything the Flyers could have done, just as it was an advantage for the 11 other teams that played opponents who traveled long distances hours before taking the ice on the 27th.
In any event, the Predators blew the doors off the Flyers in the game, and that had nothing to do with travel, so it's all kind of moot. The NHL could decide to fine the team for breaking the rules, but we'll see about that. Hopefully next year they'll consider changing the CBA to allow travel on the 26th.