Tefillin are a pair of black leather boxes containing verses from the Torah, which are bound to the arm and forehead with leather straps during morning prayers by Jewish men. This practice is a daily commandment found in the Torah, specifically in the book of Deuteronomy, which instructs Jewish people to “bind these words as a sign upon your hands and between your eyes” (Deuteronomy 6:8) in the opening lines of the text making up the Shema.
However, one notable exception to the daily use of tefillin is the observance of Shabbat, the Jewish day of rest, along with Biblical holidays like Rosh Hashanah and Pesach. Shabbat, which occurs from Friday evening to Saturday night, is a time for Jews to set aside their regular, weekday work and dedicate themselves to prayer, study, and rest. The prohibition of certain types of work on Shabbat, as outlined in the Torah, includes various creative and labor-intensive tasks, and it is considered a time of spiritual renewal and connection to God. This however is not why tefillin are not worn, or even allowed to be moved over Shabbat.
The reason why people do not wear tefillin on Shabbat is rooted in the interpretation of the commandment and the nature of Shabbat itself. The Torah instructs the binding of tefillin referred to as an, “ot” which means as a “sign” as an embodiment of the covenant between God and the Jewish people, but so too is Shabbat called a “sign”. Shabbat, as a day of rest and reflection, is meant to be a living embodiment of the Jewish faith so profound that the physical sign of tefillin becomes unnecessary.
There is a third covenant, brit milah (covenant of circumcision), sometimes known simply as the bris. This is permanent covenant is not a contradiction to what was mentioned above. It is rather the case that these symbols come as “witnesses” of the relationship between God and the Jewish people. In a Jewish court (beit din), two witnesses are needed to verify a testimony, but having a third would indicate one other is not sufficient. On Shabbat, the day itself serves as a powerful “ot,” or sign, of the covenant between the Jewish people and God, and in honoring that, Jews do not wear tefillin.
In summary, the mitzvah to wear tefillin does not extent during Shabbat and biblical holidays based on both of them too being a covenant. Shabbat is a day when the Jewish people turn their hearts and minds to God, and the absence of tefillin is not a loss, but trading one symbol of the unique Jewish relationship with God for another.