Have you ever wondered about the weird grammar of "God Rest Ye Merry, Gentlemen"? This thread is a great explanation.
(Though Mary Norris is wrong about "rest" being an imperative; it's a third-person subjunctive that acts a lot like an imperative.)
Imperatives are always in the second person: "Clean up your room" or "Take a nap" or the like. The subject is an implied "you".
But the subject of the song title is "God", which is in the third person and would normally take a present-tense verb ending in -s: "God rests."
The present subjunctive drops the -s and gives it a meaning somewhat equivalent to "May God rest." It's a lot like an imperative semantically, but it's distinct.
