The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Re’eh (17; listen to audio recording for precise citation), presents several guidelines concerning somebody who decides to accept upon himself the custom to fast on certain occasions. For example, a person might wish to accept upon himself to fast every Ereb Rosh Hodesh or during the period of “Shobabim” (the weeks when the first six to eight Parashiyot of the Book of Shemot are read), or during the Yamim Nora’im. The Ben Ish Hai writes that a person who makes such a commitment must verbally declare that he does not accept this commitment upon himself as a formal Neder (vow), or as a permanent Minhag (custom). Instead, he should state that whenever he decides to observe a fast, he will formally accept upon himself the fast on the previous day. Then, if he ever decides, for whatever reason, not to fast on one of these occasions, he is permitted not to fast. For example, if somebody decided to begin fasting on Ereb Rosh Hodesh and ensured to make the aforementioned stipulation, and one month he chooses not to fast, he simply does not make the formal acceptance the previous day. He may then eat as usual on Ereb Rosh Hodesh. This applies even if there is no particular reason for him not to fast. Since he made a point of affirming that he does not formally accept this practice upon himself as a permanent custom, he may decide not to fast whenever he chooses.
If, however, a person began observing such a practice without making any specific stipulation, then he may not discontinue the practice unless he performs Hatarat Nedarim (the formal annulment of the vow in the presence of three men). When declaring his regret for accepting this practice, he must ensure to express regret only for neglecting to stipulate that he does not accept the practice as a permanently binding custom. He must not express regret for the fasts that he had observed, for one who regrets performing a Misva loses credit, so-to-speak, for that Misva. He should therefore regret the nature of the acceptance, and not the admirable practices that he observed.
If a person mistakenly thought that a certain action was forbidden, and therefore refrained from performing the given act, then he does not require Hatarat Nedarim upon discovering that the act is permissible. For example, if he attends a Torah class and is informed that the act in question is not forbidden, he may perform that act despite the fact that he had until now followed the custom to refrain from that act. Since this custom was the result of an erroneous assumption, the entire acceptance is considered null and void, and he is therefore not bound by it. The Ben Ish Hai adds that this applies even if one thought that a certain action constituted a grave transgression, and in reality was something that some people avoid as a “Humra” (added level of stringency which is not required according to strict Halacha). Here, too, his custom to refrain from the action was undertaken based on a mistaken assumption, and therefore is not binding upon the individual at all.
Summary: One who begins observing a certain voluntary religious practice – such as fasting every Ereb Rosh Hodesh – should verbally stipulate that he does not accept it upon himself as a formal vow. Otherwise, he is bound to observe the custom unless he performs Hatarat Nedarim. One who refrained from a certain action because he mistakenly thought it was forbidden by Halacha does not require Hatarat Nedarim when he discovers that it is permissible.
