Shabbos and Yom Tov:
One is not required to wear Tefillin on Shabbos or Yom Tov and it is even forbidden to do so. One who dons Tefillin on Shabbos or Yom Tov for the sake of the Mitzvah transgresses the Biblical prohibition of “Baal Tosef/Do not add” and is belittling the sign of Shabbos and Yom Tov and is thus liable for death. It is however permitted to wear Tefillin not for the sake of the Mitzvah but for some other purpose, such as one who found lost Tefillin in an area without an Eruv and desires to wear it until he reaches a secure area.
If after accepting Shabbos one remembered he did not put on Tefillin, may he do so?
If one has not yet Davened Kabalas Shabbos, and it is prior to sunset, then he may put on the Tefillin. This applies by both Tefillin of Rashi and Rabbeinu Tam, and a blessing may be recited on the Rashi Tefillin. If however he already Davened Kabalas Shabbos, then some write that he may no longer wear the Tefillin.
 The reason: The purpose of wearing Tefillin is to serve as a sign between us and Hashem. This is learned from the verse “And it shall be for you a sign”, [which teaches that Tefillin are donned only when they alone are a sign and not when something else is also a sign]. Now, on Shabbos the sign of Shabbos is present between us and Hashem, and hence one is not required to wear Tefillin on Shabbos. Furthermore it is even forbidden to do so due to Baal Tosif and it belittling the sign of Shabbos. [ibid]
 Piskeiy Teshuvos 31/1 based on Taz 600/2 regarding Shofar
 The reason: As one does not intend to accept Shabbos to the point that he can’t fulfill the Mitzvah of Shofar. Likewise if the Sages permit doing a Mitzvah during Bein Hashmashos then certainly they permit it before Bein Hashmashos, after he accepted Shabbos. However a blessing may not be made as this shows that he is doing so for the sake of the Mitzvah and it hence carries the decree of Raba that one should not blow Shofar for the Mitzvah on Shabbos. [Taz ibid]
 Piskeiy Teshuvos ibid footnote 3 that this applies even according to the lenient opinion in Admur 30/5 as here there is a prohibition according to all to put on Tefillin on Shabbos.; See however Taz ibid that one may blow Shofar even after Maariv