Salve!
This is the kind of thing for which we don't really have any evidence in authentic Latin, so we're left guessing somewhat.
I've usually seen (in modern use, that is) this sort of expression in the accusative, because
vobis/tibi opto, "I wish you..." is implied. So
felicem Nativitatis diem (vobis/tibi opto) or why not even more simply
felicem Nativitatem (vobis/tibi opto).
Now I suppose one could still defend the nominative by saying they're implying
tibi/vobis sit...
Edit: Actually, I wonder how it is in modern inflected languages. How is it in Czech, for example,
Godmy ? Or in Russian,
Quasus ? Do you express "Merry Christmas" or "Happy birthday" as nouns in the accusative?