Void Where Prohibited
Kol Nidrei, the well-known Yom Kippur tefillah, should not actually be said on Yom Kippur, according to the Rema. However, there is some debate as to the exact reason.
The Gemara says we do not convene a beis din to perform hataras nedarim on Shabbos. However, if the vow one wants annulled inhibits one from enjoying Shabbos, then we can convene a beis din to permit that vow even on Shabbos. Certainly if one vowed not to eat or drink, that vow may be annulled on Shabbos. And the Mishnah Berurah writes that even a vow not to eat meat or drink wine may be annulled on Shabbos. Similarly, the Mishnah Berurah says that a vow not to sleep on Shabbos significantly impacts one’s enjoyment of Shabbos. Therefore, we may convene a beis din to annul that vow.
Other poskim offer other examples of vows that can be annulled on Shabbos. Shu’T Yehudah Yaaleh writes that one can annul a vow not to eat challah on Shabbos. Even though one can use alternative breads such as rye, pumpernickel, or perhaps hamburgers buns, challah is accepted as the bread of Shabbos.