Joel 1:14

We have observed abundance of tears shed for the destruction of the fruits of the earth by the locusts; now here we have those tears turned into the right channel, that of repentance and humiliation before God. The judgment was very heavy, and here they are directed to own the hand of God in it, his mighty hand, and to humble themselves under it. Here is, I. A proclamation issued out for a general fast. The priests are ordered to appoint one; they must not only mourn themselves, but they must call upon others to mourn too: "Sanctify a fast; let some time be set apart from all worldly business to be spent in the exercises of religion, in the expressions of repentance and other extraordinary instances of devotion." Note, Under public judgments there ought to be public humiliations; for by them the Lord God calls to weeping and mourning. With all the marks of sorrow and shame sin must be confessed and bewailed, the righteous of God must be acknowledged, and his favour implored. Observe what is to be done by a nation at such a time. 1. A day is to be appointed for this purpose, a day of restraint (so the margin reads it), a day in which people must be restrained from their other ordinary business (that they may more closely attend God's service), and from all bodily refreshments; for, 2.

— Matthew Henry's Concise Commentary (public domain)

Sanctify . . . a fast--Appoint a solemn fast. solemn assembly--literally, a "day of restraint" or cessation from work, so that all might give themselves to supplication (Joe 2:15-16; Sa1 7:5-6; Ch2 20:3-13). elders--The contrast to "children" (Joe 2:16) requires age to be intended, though probably elders in office are included. Being the people's leaders in guilt, they ought to be their leaders also in repentance.

— Jamieson-Fausset-Brown Commentary (public domain)