We are beginning siman 9. The Chayei Adam writes that it is inappropriate to enter Shabbos feeling afflicted. Therefore, one should not fast on erev Shabbos. One should feel the enjoyment of Shabbos already from the beginning of Shabbos. The Chayei Adam clarifies that the only exception is a person with a delicate constitution who, if they eat on erev Shabbos, will not be able to eat the Shabbos seudos.
We learned that it is appropriate to enter into Shabbos with an appetite, and to even skip a meal if one has that constitution (S0024). However, even if one is strong enough to skip one meal, it is inappropriate to fast the entire day, because the vast majority of people will find it uncomfortable and enter into Shabbos in a state of affliction (with the exception of the person with a delicate constitution discussed above).
When the Chayei Adam discusses fasting on erev Shabbos, he is referring both to personal fasts undertaken by an individual and public fasts. The only one of the public fasts which falls out on erev Shabbos is Asara B’teves.
In general, we know that a taanis is not over with sunset but with tzeis hakochavim, nightfall. If one is fasting on erev Shabbos, if they are to wait until nightfall to break their fast, it comes out that they will be fasting on Shabbos, because they accept Shabbos with sunset (or even earlier) but do not break their fast until nightfall. Therefore, the Chayei Adam writes that if one undertakes a personal fast on erev Shabbos, they should verbalize a stipulation at the time that they undertake the fast that they will not complete the fast until nightfall. Bedieved, even if the person did not make such a stipulation, they do not need to wait until nightfall. As soon as they are ready to make kiddush, they are allowed to eat. This applies even if one accepted Shabbos at plag.
Regarding the calculation of nightfall, since fasting is derabanan, we use an earlier calculation to determine tzeis hakochavim. As it becomes dark, the larger stars become visible first, then medium sized stars, and finally, as more light leaves the sky, the small stars. The stars required to indicate the end of a fast are medium stars. The stars required to indicate the end of Shabbos are small stars.
The Gemara mentions that it is appropriate to fast on the yahrzeits of a father or mother. The Shulchan Aruch brings this idea as an appropriate minhag, although many no longer follow this minhag due to their weaker constitution. We will discuss the appropriate practice when the yahrzeit falls out on Friday in the upcoming shiur, be’ezras Hashem.
The Kitzur Shulchan Aruch, in siman 32, discusses various health suggestions. He writes that it is appropriate to skip one meal during the week, in order to give a person’s stomach a chance to rest. He suggests that the best time to implement this practice is erev Shabbos. His suggestion is a compromise in that they are not fasting entirely, so they are not feeling uncomfortable, but are skipping a meal so that they enter Shabbos with an appropriate appetite.
One should not fast on erev Shabbos. If they accept a personal fast on Friday, they should stipulate that they will break it when they are ready to accept Shabbos (even if they accept Shabbos at plag). Even if they did not make this stipulation, their fast is still over when they accept Shabbos.
However, as we have learned, it is appropriate to skip a meal–without fasting the entire day–in order to enter Shabbos with an appetite.
Click here to watch Rabbi Reingold performing a Model Seder.