The custom of the three elements, fasting, sackcloth and ashes, are an ancient eastern custom to show humiliation and repentance. It was neither invented nor originated by the Israelites but was common in the ancient east. We see this often in many places, even in Job who was before Moses and thus was not a Hebrew. The practice lasted into NT times. Here are some examples:
- Job 16:15 - "I have sewed sackcloth over my skin and buried my brow in the dust.
- Est 4:1 - When Mordecai learned of all that had happened, he tore his clothes, put on sackcloth and ashes, and went out into the middle of the city, wailing loudly and bitterly.
- Isa 58:5 - Is this the kind of fast I have chosen, only a day for people to humble themselves? Is it only for bowing one's head like a reed and for lying in sackcloth and ashes? Is that what you call a fast, a day acceptable to the LORD?
- Dan 9:3 - So I turned to the Lord God and pleaded with him in prayer and petition, in fasting, and in sackcloth and ashes.
In commenting on Jonah 3:5, the Jamieson-Fausset-Brown Bible Commentary notes this:
fast … sackcloth—In the East outward actions are often used as
symbolical expressions of inward feelings. So fasting and clothing in
sackcloth were customary in humiliation. Compare in Ahab's case,
parallel to that of Nineveh, both receiving a respite on penitence
(1Ki 21:27; 20:31, 32; Joe 1:13).
Similarly, Matthew Poole observes:
Proclaimed a fast; every one called upon other to fast, of cried out
it was high time to fast, repent, and supplicate God, so some think;
but this passage is an anticipation, tells us what was done, and will
tell us afterwards on what grounds, authority, and example it was
Put on sackcloth; a ceremony very usual in mournings, private or
public, in those countries, and a token of their true mourning; this
all did, great and small, rich and poor.
Similarly Gill observes:
and proclaimed a fast; not of themselves, but by the order of their
king, as follows; though Kimchi thinks this was before that:
and put on sackcloth, from the greatest of them even to the least of
them; both, with respect to rank and age, so universal were their
fasting and mourning; in token of which they stripped themselves of
their common and rich apparel, and clothed themselves with sackcloth;
as was usual in extraordinary cases of mourning, not only with the
Jews, but other nations.
Similarly the Pulpit Commentary has this:
So they believed in God, and proclaimed a fast. Spontaneously, without
any special order from the authorities. Before the final fall of
Nineveh, the inscriptions mention, the then king ordered a fast of one
hundred days and nights to the gods in order to avert the threatened
danger (see a note by Professor Sayce, in G. Smith's 'History of
Babylon,' p. 156). Put on sackcloth (comp. Genesis 37:34; 1 Kings
21:27; Joel 1:13). The custom of changing the dress in token of
mourning was not confined to the Hebrews (comp. Ezekiel 26:16).