Tanya for 11 Tamuz

However, all this (4) applies to the strong and robust, whose physical health would not be harmed at all by repeated fasts, as in the generations of yore.

But whoever would be affected by many fasts, and might thereby suffer illness or pain, G-d forbid, as in contemporary generations, is forbidden to undertake numerous fasts, even for sins punishable by excision or execution, and certainly not for [violation of] the positive and prohibitive commands that do not involve excision.

Instead [the measure of fasting is] the personal estimate of what will not harm him at all.

For even in those early generations, in the times of the Tannaim and Amoraim, only the robust who could mortify themselves fasted so frequently.

But whoever cannot fast yet does so, is called a "sinner" in Tractate Taanit, chapter 1. (5)

This applies even to one who fasts for specifically known sins, as Rashi explains there.

[The very same author of the opinion that he who fasts frequently is considered "holy", because (as Rashi explains) his sins are thereby expunged, goes on to state that he is considered a sinner if he cannot fast yet does so].

And it is written in Tractate Zevachim, chapter 1, (6) that "there is no one of Israel who is not guilty of [transgressing] a positive commandment....."

[Thus, though there are always sins for which one should fast, one should do so only if this will in no way impair his health; otherwise, he is considered a sinner], especially if he is a student of Torah, in which case he is doubly punished, for the weakness resulting from his fast prevents him from studying Torah properly.

What, then, is his remedy?

[I.e., what is such a person to do in order to be beloved by his Creator as he was before his sin?]

He should comply with the verse that says, (7) "Redeem your sin with charity."

And, indeed, the codifiers of Torah law specified that one should donate the equivalent of eighteen [large Polish] coins called "gedolim Polish" for each day of repentance.

The wealthy should add to this [amount for the redemption of each fast-day] according to his means, as stated in Magen Avraham in the Laws of Fasts. (8)

4. Note of the Rebbe Shlita: "Concerning all the laws about to be stated, see Shulchan Aruch, Orach Chayim 571 and its commentaries; Shulchan Aruch Admur HaZaken, Hilchot Nizkei Guf veNefesh, Sub-section 4."
5. Note of the Rebbe Shlita: "End of p. 11a; see also Rambam, Hilchot Deot, beg. of ch. 3, and commentators ad loc."
6. 7a, and see Rashi there.
7. Daniel 4:24.
8. Shulchan Aruch, Orach Chayim 568:12, and commentaries.