It is clear that this verse continues God’s response begun in verses 3b-4 by repeating the words fast, day, and humble. Here God asks the people whether their idea of fasting and his are comparable.

Is such the fast that I choose, a day for a man to humble himself?: God asks whether the sole purpose of fasting is for a person to inflict hardship on himself or herself. The implied answer is “No!” If depriving oneself is the only purpose of fasting, it achieves nothing. It is not what God intends. Is such the fast introduces the subject of the question. It refers to the kind of fasting the people have been doing. This expression in Hebrew implies that there is something wrong with their fasting. New International Version expresses it adequately with “Is this the kind of fast.” The verb choose may be rendered “require” (Revised English Bible) or “desire” (New Jewish Publication Society’s Tanakh). One commentator suggests “favor.” Good News Translation combines this verb with the word acceptable at the end of the verse, saying “be pleased with.” A day for a man to humble himself describes the people’s fasting. As in verse 3, a day refers to the time of fasting, not just one day. The Hebrew word for man (ʾadam) is generic and refers to all human beings. For humble himself, see the comments on “humbled ourselves” in Isa 58.3. Possible alternative models for this whole question are “Is merely depriving oneself really the kind of fasting that I favor?” and “If a person does nothing else but deprive himself/herself, is that really the kind of fasting that I want?”

Is it to bow down his head like a rush, and to spread sackcloth and ashes under him?: Here God asks whether fasting is merely an outward expression of bowing one’s head and sitting on sackcloth and ashes. The implied answer is “No!” The pronoun it in this second question refers to the same subject as the previous one, namely, the kind of fasting that God desires. For a person to bow down his head shows sadness or sorrow. Here it is compared to the way a rush stands with its top part drooping. A rush or a “bulrush” is a plant that grows beside a watercourse (see the comments on 9.14, where the Hebrew word for rush is rendered “reed”). Translators may choose another plant that bends over; for example, Good News Translation has “blade of grass.” If none are available, translators may begin this question by saying “Is the kind of fasting that I expect from people a mere drooping of their heads to show how sad they are…?” Sackcloth was usually worn as clothing in lament rituals (see the comments on 3.24), but here the person who is fasting places it on the ground and sits on it. Ashes were also used in lament rituals. Sometimes people placed the ashes on their heads (see 2 Sam 13.19), or they sat on them, as here and in Job 2.8.

Will you call this a fast, and a day acceptable to the LORD? is another rhetorical question. God challenges the people to consider whether their fasting is genuine fasting. The demonstrative pronoun this refers to their outward ritual fasting just described. The verb call may be rendered “consider” or “regard.” The noun day refers to both the time and all the activities related to fasting, so it may be rendered “fasting.” The Hebrew word translated acceptable describes something that will win favor or meet expected requirements. As noted above, Good News Translation combines this word with the verb choose, saying “be pleased with.” Good News Translation also changes the third person reference to the LORD to first person, because he is speaking. Other languages may find this helpful (compare verse 2). This whole question may be rendered “Do you think that kind of fasting is what the LORD wants [or, I want]?”

Two translation examples are given for this verse. The first one keeps the rhetorical questions, while the second one changes them to strong statements.

• “Is that the kind of fasting that I require,
a period of self-denial?
Is it intended as a time of bowing the head,
bowed down like a bulrush,
as you sit on sackcloth and ashes?
Is that what you consider fasting
that is acceptable to me?

• “I do not demand this kind of fasting,
a time when people deprive themselves.
Fasting is not a mere bowing of the head,
when the head bows over like a drooping plant.
It is not merely sitting on sackcloth and in ashes.
You cannot call that a fast;
that is not the kind of fasting I require.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .