We are well conscious of the fact that we do not don tefillin on Shabbos. In fact, the word שבת is an abbreviation for פליןתמקום בבת ש. What is the reason for this?
1) Tosafos tells us from a Midrash
that the Egyptians forced us to work with the 39 forms of labor. These were the 39 melachos. For this reason, at Kidush on Shabbos, we say זכר ליציאת מצרים. Tefillin binds us to Hashem, as it says וקשרתם, to bind. Consequently, we don’t wear Tefillin on Shabbos since we already have a reminder of Yetzias Mitzrayim, namely Shabbos.
2) The Avudraham provides us with a different reason
There are three mitzvos that are described as an אות, sign — שבת, ברית, תפלין (whose initials spell out the word שבת). These three mitzvos are a sign and testimony to the fact that we are servants to Hashem. Since על פי שנים עדים...יקום דבר, according to two witnesses...a matter is confirmed, any Jew is only complete if he has two witnesses that he is a Jew. Therefore, on Shabbos (and Yom Tov) which is called a sign, we don’t wear tefillin since we already have two signs — Shabbos and bris.
The question is asked if one is lacking the sign of bris or Shabbos (such as one who is in the army and must desecrate Shabbos or one who was never circumcised because of medical reasons), should he put on tefillin on Shabbos so that he could have two signs? The answer is that he should not don tefillin on Shabbos.
