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 Shabbat Parashat Acharei Mot| 5763Ask the RabbiQuestion: If one is supposed to fast on a certain day and mistakenly eats, does he need to continue his fast or does it not pay since he anyway didn’t fast?
Answer: We should first understand the conceptual basis of fast days, explore distinctions between different fast days and then answer your question.
One element of fasting highlighted in your question is the mitzva of going a day without eating. When this is the only element of a fast day, we indeed do say that once one has eaten, there is nothing more to lose. (Please note that eating, in this context, means eating a c’zayit within k’dei achilat pras (roughly, one sitting) which is a full violation of the fast. Even though it is forbidden to eat any amount on a fast day, one who just takes a small taste and/or spits out the food has not fully broken the fast and must certainly continue (Mishna Berura 568:5)). If one makes a vow to fast a day but does not incorporate the date of the fast in his vow, then there is no purpose to continue, as this day will not count toward fulfilling his vow in any case. Similarly, some explain the idea that a firstborn who partakes in the celebration of a siyum on Erev Pesach may eat the whole day, based on the assumption that ta’anit bechorot was accepted with only the aforementioned dimension (Eretz Hatzvi, cited in Minchat Yitzchak VIII, 45).
A second element of some fast days is the prohibition to eat. On Yom Kippur, there is certainly a prohibition to eat, above and beyond the mitzvah to fast (Pesachim 36a provides one of many applications of this idea). Thus, just as one who violates Shabbat may not continue doing so, so too, one who ate on Yom Kippur may not continue eating. The Shulchan Aruch (Orach Chayim 568:1) rules that whenever the day one fasts has a specific significance, one who eats cannot decide to switch the date after failing to fast the whole day, since fasting on a future day does not replace the need to fast on this day. He applies this logic to the four principal, rabbinic fast days, one who fasts on a yahrzeit, and one who specifies even an arbitrary day in his vow to fast. The same logic applies when one takes part in the fast of “Behab,” “Yom Kippur Katan,” or any, even optional, public fast which is set for a given day.
Whether one is required to fast another day to make up for not successfully fasting on the appointed day is a somewhat complicated question, beyond the scope of this response. You can start your research with the Rama 568:1 and the Biur Halacha, ad loc.
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