Let's say I make a neder not to benefit from the five grains - wheat, barley, rye, oat(?), and spelt. These are the only grains which can be used for Matzah on Pesach (Pesachim 35b). Assuming that a neder cannot override a mitzvah d'Oraisa (that is, according to the Ramban cited there), is my neder binding?
- On the one hand, I can get away with that for most of the year without violating any absolute obligations, so maybe the one week a year on which it's mandatory is batel b'rov.
- On the other hand, maybe a neder needs to be able to never be against a mitzvah, and this would be against that rule.
- On the third hand, maybe it would be valid whenever it can be valid, and not whenever it isn't. Therefore, it would be valid during the year, but not on Pesach.