December might be too late. I know from last year, scores came out December 22. They have to send the score after that.
Plus you can't apply ED or EA if you take something in Dec.
November is also late for EA and ED, but it should be fine for RD.
October is the final chance for most seniors who apply ED or EA.
Early Decision - Early Action - apply to colleges, search
Sometimes, students who apply under these plans have a better chance of acceptance than they would through the regular admissions process. These plans are also good for colleges, because they get students who really want to go to the school to commit early in the process.