קבלה אויף שמות 31:13

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.”