Jamieson, Fausset and Brown on Joel 1:14

14. 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; 1Sa 7:5, 6; 2Ch 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.