I don't know why so many people believe it is illegal to fire someone over the phone. It is not. In any state.
No Federal law and no law in any state dictates to the employer what method of communication can be used to fire someone. You can be fired by phone, fax, text message, e-mail, ASL, telegram (singing or otherwise), IM, skywriting (as the other poster suggested - I like that one!) or smoke signal. Or any other method of communication you can think of.
Nor does having it happen on the day off have any legal significance.