Yoel 2:15

15 Blow the shofar in Tziyon, sanctify a tzom, call an atzarah (solemn assembly);

Yoel 2:15 Meaning and Commentary

Joel 2:15

Blow the trumpet in Zion
For the calling of the people together to religious duties, which was one use of the silver trumpets made for and blows by the priests, ( Numbers 10:2 Numbers 10:3 Numbers 10:8 Numbers 10:10 ) ; sanctify a fast, call a solemn assembly; (See Gill on Joel 1:14).

Yoel 2:15 In-Context

13 And rend your lev, and not your garments, and turn unto Hashem Eloheichem; for He is channun (gracious) and rachum (merciful), slow to anger, and of rav chesed, and He relents from sending calamity.

14 Who knoweth but that He will return and have pity, and leave behind Him a brocha; even a minchah and a nesekh unto Hashem Eloheichem?

15 Blow the shofar in Tziyon, sanctify a tzom, call an atzarah (solemn assembly);

16 Gather the people, set apart as kodesh the kahal (congregation), assemble the zekenim, gather the olalim, even those nursing at the breast; let the choson go forth of his cheder, and the kallah out of her chuppah (canopy).

17 Let the kohanim, the mesharetim of Hashem, weep between the Ulam and the Mizbe’ach, and let them say, Spare Thy people, Hashem, and make not Thine nachalah (inheritance) an object of scorn, a byword among the Goyim. Why should they say among the peoples, Where is their G-d?