December 7th is the deadline by which you have to offer arbitration to your former players who are now free agents (Burnitz, Nomar, Fox, Grieve). If you don't offer, you wave goodbye to them. December 20th is the deadline by which you have to tender a contract offer to all of your other players - those who are eligible for arbitration (Zambrano, Patterson, Hairston, Macias) and everyone else. If you don't tender them a contract offer, they become free agents.