2,751. Taking Vows About Mitzvos
Hilchos Nedarim 3:5
Let’s say that someone’s father or teacher died on a certain day; he took a vow to fast that day and he did so. Years later, he says, “Let this day be like the day on which my father/teacher died.” In such a case, eating on that day becomes prohibited. He has linked this day to his existing vow, rendering it prohibited like the day that is already prohibited to him. The same is true in all comparable cases.
Hilchos Nedarim 3:6
Vows taking effect vis-à-vis mitzvos is as follows: If someone declares matzah to be prohibited to him on Passover night, dwelling in a sukkah to be prohibited to him on Succos, or picking up tefillin to be prohibited to him, these things become prohibited to him. Accordingly, if he eats matzah, dwells in a sukkah or picks up tefillin, he’s liable to the penalty of lashes; the same is true in all comparable cases. It should go without saying that if someone says, “I am obligated in an offering if I eat matzah on Passover night,” that he must bring an offering. Again, the same is true in all comparable cases.