MM, What's wrong with saying "thanks for the invite. However, I can't attend." If that sounds too formal, then how about "I can't. Thanks for the invite." I doubt xbf cares why. Then sashay off and enjoy the day.
3 kids BD 12/15/13 (IDKIILY. ) Rope dropped Cirque du Soleil style D final 9-9-14 "Some people are born on 3rd base and go through life thinking they hit a triple." Barry Switzer