Tefillin can be worn on weekdays throughout the day. On Shabbos, however, there is no mitzvah to wear tefillin, and doing so is a transgression of the prohibition of bal tosif. Even if one does not intend to be yotzei the mitzvah, laying tefillin on Shabbos is forbidden mid’rabonon. There is a concern that one may remove them while walking outside and end up carrying them in a reshus harabim. Some maintain that there is no such prohibition. As in the case of Shabbos, one should not put on his tefillin at night. The wearer may fall asleep and fail to maintain the bodily cleanliness which is necessary when wearing tefillin.
[שו"ע כט, א, ומשנ"ב א-ב; ביאורים ומוספים דרשו, 2 ;סימן ל, ס"ק א]