Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 31 Laws of Tefillin on Shabbat and Yom Tov
“ Paragraph 2 — On Chol HaMoed it is prohibited to wear tefillin for the same reason, namely that Chol HaMoed is also an אות (sign) . Rem"a: There are those who maintain that one is obligated to wear tefillin on Chol HaMoed [Beit Yosef quoting the Rosh] and the custom in all these regions is to put on tefillin with a beracha. ”
