What should someone wear on Chol Hamoed?
The Mechilta (12:16) brings a verse in Vayikra (23:2) to teach us that on Yom Tov and even on Chol Hamoed, one is obligated to sanctify the day with clothes and food.
However, the dress code on Chol Hamoed differs from the other days of Yom Tov. Chayeh Adam (106:1) rules on Chol Hamoed that one should wear Shabbos clothes, which are inferior to Yom Tov clothes like we find in Shulchan Aruch (529:1).
According to the Mishnah Berura (530:1, S.T: 530:4) and Aruch HaShulchan (530:4), the obligation on Chol Hamoed is only to dress better than on a regular weekday, without the necessity of Yom Tov or Shabbos clothes.
Shulchan Aruch HaRav (529:5) permits wearing weekday clothes on Chol Hamoed.