Дварим 6:8 Каббала: Sha'arei Orah

Now, because this quality must necessarily be bound to the Divine qualities that are above it, so that their beneficence and blessings can come to it, HaShem-יהו״ה, blessed is He, granted the Jewish people the strength and power to affect this bond between this quality and those that are above it. This is the mystery of the phylactery of the hand-Tefillah Shel Yad-תפלה של יד.” It is about this that the verse states,¹⁹⁵Deuteronomy 6:8 “Bind them as a sign upon your arm and let them be as ornaments between your eyes.” Thus,¹⁹⁶Talmud Bavli, Brachot 14b “whosoever recites the morning Shema recital without donning phylacteries-Tefillin, it is as if he is testifying falsely against himself.” There is therefore no way by which his prayer can enter, since the prayer-Tefillah-תפילה is dependent upon the phylactery-Tefillah-תפילה.

However, on Shabbat and Holidays prescribed by Torah, the donning of phylacteries-Tefillin is not necessary. This is because, on those days, there are several other wondrously great matters and all the gates are open. This is analogous to the fact that when a person has the king himself, he has no need for the king’s signet. Thus, Shabbat and the Holidays, which themselves are called a “sign-Ot-אות,”¹⁹⁸Exodus 31:13 and elsewhere. are sufficient, so that on those days we do not need to¹⁹⁹Deuteronomy 6:8 “bind them as a sign-Ot-אות.”