Translation:Shulchan Aruch/Orach Chaim/31
31: Law of Tefillin on Shabbat and Yom Tov
1 It is prohibited to wear tefillin on Shabbos and Yom Tov since they themselves are a "sign" and if one were to also wear on them an extra sign it would be a disgrace to their sign.
2 On Chol HaMoed it is also prohibited to wear tefillin for this same reason, namely that the days of Chol HaMoed are also a "sign". Gloss: And there are those who say that one is obligated to wear tefillin on Chol HaMoed [Beit Yosef quoting the Rosh] and such is the custom in all these regions to put them on on Chol HaMoed with a beracha. However, the beracha is not recited out loud in shul like the other days of the year.