Why tefillin are not worn on Shabbos and Yom Tov
Shabbos, Yom Tov (including Yom Tov sheni in chutz la'aretz) and tefillin are all referred to as ois. Wearing the ois of tefillin on Shabbos or Yom Tov would detract from the special status of these days , and doing so is forbidden. Wearing tefillin on these days for the sake of the mitzva is also a violation of ba'al tosif. Mid'rabonon, one may not wear them on these days even if they are not being worn for the sake of the mitzva. The poskim discuss whether they may be worn in private if they are not being worn for the sake of the mitzva.
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