Why do we not put on Tefillin on Shabbos?
Why is one not allowed to put on Tefillin on Shabbos?
And similarly, why is Tefillin Muktzah?
It is prohibited to put on tefillin on Shabbos and Yom tov, since tefillin are called אות (Shemos 13:9), and so is Shabbos (Shemos 31:17). Thus, donning tefillin is disrespect (זלזול) to the אות of Shabbos. (Shulchan Aruch, OC 31:1)
There is discussion amongst the poskim if, because tefillin are assur to wear, they should fall into the category of k'li shem'lachto l'issur (object designed for a prohibited use) on Shabbos. Mishna Berura (31:2) rules that one should consider them muktza, but may move them if necessary (בשעת הדחק), such as if they might fall on the ground or be stolen.