לזכות ידידי אהובי ר' הירש יונתן בן ר' מרדכי אפרים הוא וביתו וכל אשר לו לברכה והצלחה בכל מעשי ידיו
There is a fascinating discussion in the poskim about wearing tefillin on Chol Ha-moed. There is a vast corpus of literature and I am not going to give an overview here. Suffice it to say that it is a machlokes. The Rema [along with many others] says that one should wear tefillin on Chol Ha-moed while the Gra [whose yahrtzeit is today] says that one should not. The Zohar Ha-kadosh says that one who lays tefillin on Chol Ha-moed is ... חייב מיתה.
Whoa!
The minhag in Eretz Yisrael is to NOT put on tefillin. The Mishna Brura [and many others] say that even a ben chutz la-aretz should not wear tefillin in shul in a place where the minhag is not to wear tefillin because of לא תתגודדו - not to have different halachic practices, making the Torah appear as two Torahs [there are oceans of material discussing the paramaters of this law]. Others [such as Rav Ovadiah Yosef] disagree and allow everybody to follow their own minhag [either because when it is known that it is a machlokes there is no לא תתגודדו or because regarding minhagim there is no לא תתגודדו or because when one is dealing with a bitul mitzva di-oraisa you don't pay attention to what others are doing]. I have seen a lot of tefillin this Chol Ha-moed and it bothers me. But who asked me anyway...:-).
Rav Chaim Soloveitchik didn't wear tefillin on Chol Ha-moed. He was asked by his son Rav Moshe why he is not חושש for ספק דאורייתא לחומרא and maybe he must wear tefillin. Rav Chaim replied that there is no safek at all because the gemara in Erchin [10] says that Chol Ha-moed has kedushas Yom Tov and a person would thus be absolved from the obligation to wear tefillin.
