In California for an involuntary term you must receive your final check, including any accrued but unused vacation time, on your last day of work. So if you were fired on December 14, your final check was due on December 14.
If this did not happen, call them and ask politely (you catch more flies with honey than vinegar) when you will be receiving your last check. If the answer is not acceptable, tell them that you are sorry but since CA law states that you should have received your check on the day you were fired, you will have no recourse but to call the California Department of Labor.
Then do it. The CA DOL doesn't mess around.