Many years ago, my principal told us before succos bein hazmanim,
One who does not wear shabbos clothing (i.e. a white shirt, suit and tie etc.) on Chol HaMo'ed - specifically when he comes to shul - has disgraced the festival (i.e. Succos) has no share in the World to Come.
I never understood where my principal got this from. However, a few weeks ago I was looking in Avos and where the mishnah says (3:15),
One who disgraces the festivals... has no share in the World to Come.
What I want to know is, how did my principal come to the conclusion that if one doesn't wear his shabbos clothing when he comes to shul on chol hamo'ed, he has disgraced the festival?
Is this connection between shabbos clothing and "disgracing the festival" brought down in any halacha sefarim? (i.e. Mishnah Berurah, Kitzur Shulchan Aruch or Shulchan Aruch)