Not only does the roster freeze come into play tonight, but there will not be any games played from Christmas Eve to the 27th. Also allowing players to spend time with their families. The freeze will run from midnight tonight through December 28th.
As far as what this means for the Leafs, it basically just means that whoever is with the team when the bell strikes midnight in terms of roster players and healthy scratches, those are the players you'll see through December 27th barring injury.
It's one of the few things the league gets right in terms of scheduling. Maybe next year they will figure out how to stagger games and their start times correctly.
Also, for those who are worried about a gap in hockey, the World Juniors are set to begin on Boxing Day!