I think I like better felicem diem nativitatis, with something like tibi [vobis] opto being understood, as this seems more consonant with such expressions as salutem plurimam [dicit or dat]. Has anyone more learned an opinion?
I'm not sure how relatively learned I am, adspector, but you're right! I'd favour the order: felicem nativitatis diem! or perhaps: felicem Domini nativitatis diem!
I'm with Wolsey: ego et meus rexThat aside, what was the medieval latin of the republic of letters, of our forebears, and of the Church before the classicists got at it and helped to kill it dead?
felicem natalem Christi