2,753. Things That Lack Substance
Hilchos Nedarim 3:9
If someone takes a vow to fast on Shabbos or yom tov, he must fast. This is because vows take effect even regarding mitzvos as we have discussed. Similarly, if one makes an oath to fast every Sunday or every Tuesday for the rest of his life and yom tov or erev Yom Kippur falls on that day, he must fast. It should go without saying that this also applies to Rosh Chodesh. However, if Chanukah or Purim falls on this day, his vow is overridden by the holiday. Since the prohibition against fasting on these holidays is of rabbinic (rather than Scriptural) origin, it requires additional support. For this reason, such a vow is overridden by the rabbinic law.
Hilchos Nedarim 3:10
“Vows only take effect on something that has substance” (in halacha 3:1) means that if someone makes a vow saying, “My speech is to you like an offering,” he is not rendered prohibited to speak to the other person because speech lacks substance. Similarly, if he says, “My speech is prohibited to you,” it’s not like saying, “My produce is prohibited to you” or “My produce is to you like an offering” – statements that would effectively render the produce prohibited to the other person. Therefore, if one person tells another, “An offering, I won’t speak to you,” “I won’t act with you” or “I won’t go with you,” or if he said, “An offering, I won’t sleep,” “I won’t speak” or “I won’t walk,” or if one told their spouse, “An offering, I won’t be intimate with you” – in all of these cases, the vow is ineffective. This is because it’s like saying that one's speech, travel, action or intimacy is like an offering, but these things lack substance.