I have heard from friends that one should not wear Tefillin on Sukkot. Can someone find me a source?
Rav Yosef Karo (ibid :2) rules that the days of Chol HaMoed are included in this prohibition, but Rav Moshe Isserlis rules they are not included and Tefillin should be worn.
Modern customs vary widely on the latter point and one's personal rabbi should be consulted for a final ruling.
This is based on a machlokes as to whether the tefillin during chol hamoed are considered "os" in the same way as Yom Tov or not. The OU discusses this machlokes and actually ties it to the the discussions of what type of "os" (sign) the tefillin represent. Since we have special mitzvos on chol hamoed, tosfos (as an example) would say that chol hamoed is a kind of os. Others would say that since the Yomim Tovim forbid melacha and chol hamoed does not, then chol hamoed is not enough of an os to prevent us from wearing tefillin. Thus, the machlokes is indeed based on the kind of os shown by the tefillin.
There are those who would wear tefillin if they have to go to work on chol hamoed and refrain once they can stop working on chol hamoed. However, the normative minhag that I have seen is to do one or the other.
The reason we don’t put on tefillin on the holidays themselves, as well as on Shabbat, is that the tefillin are referred to in the Torah as an OT – a sign; but Shabbat and holidays are also considered a sign, so on these days we don’t display the “sign” of tefillin (Menachot 36b). What about chol hamoed?
Tosafot suggest that there are two distinct kinds of “signs” on the holidays. One kind of sign is refraining from labor, which is only on the holiday itself; the second kind is the special observances of the holidays, such as matza on Pesach and sitting in the sukkah on Sukkot, which exists also on chol hamoed.
The Beit Yosef (OC 31) cites the Zohar Chadash which also likens the two types of days to two aspects of tefillin. The Zohar Chadash refers to the holidays as the “tefillin” of HaShem: Yom Tov itself the head tefillin and the intermediate days the hand tefillin. On these days we are adorned by the tefillin of HaShem, and so it would be superfluous and even insolent to wear tefillin made by us (Zohar Chadash on Shir HaShirim, vol. IV p. 64 column d).
This distinction is closely related to that of the Tosafot. A common explanation of tefillin is that the head tefillin represent sanctity of thought, and those of the hand sanctity of deed. The Zohar likens Yom Tov, whose theme is the frame of mind of inner joy, to the head tefillin, and Chol HaMoed, with its special holiday observances, to the hand tefillin.
The Rema, who rules that tefillin are put on during chol hamoed, holds that two berakhot are said on the head tefillin, and one of these (l’haniach) exempts the hand as well. Based on the blessing, we would view the head tefillin as the primary mitzva and the primary sign. Based on the distinction of the Zohar, the difference of approach regarding the blessings is parallel to that regarding tefillin on chol hamoed (Orach Chaim 25:5; 26:2; 31:2).
The basic prohibition of tefillin on chol ha'moed seems to come from Zohar. The problem with this is that halacha is not decided according to the mystical writings.