Well, I just saw a sentence from a present-day Polish women's journal "...uroczystość TWEMU.....".
Obviously, purposely archaic diction:-)
Doesn't make any sense. It's clipped, where's the rest of it?
Indeed, clipped. But it still may be purposely archaic. The -u is the archaic ending of the singular masculine dativ (celownik) which has survived until today in several common nouns: ojc-u, brat-u, księci-u, księdz-u, ps-u, kot-u and in others. In biblical language they would say "ojcu twemu", but in contemporary Polish it would be "twojemu ojcu".