“Gather yourselves together like stubble.” Although most versions just have, “gather yourselves together,” the Hebrew verb is qashash (#07197 קָשַׁשׁ), and it was used of gathering stubble or sticks (cp. Exod. 5L7, 12; Num. 15:32-33; 1 Kings 17:10), so the translation “gather yourselves together as stubble” is justified, and certainly applicable here in Zephaniah 2:1, especially since verse 2 mentions “chaff.” When grain was harvested and threshed, the process of threshing produced three distinct products: the kernels of grain, stubble (or “straw”), and chaff. The stubble was the large pieces of the shaft, and the chaff was the tiny broken pieces of the stalk. The NET has, “Bunch yourselves together like straw.”
God’s wrath was going to be poured out with a fiery fury (Zeph. 1:18; 3:8; Mal. 4:1). The arrogant people lived in sin, so now it seems that God calls them to gather together to see if they can face God’s fiery judgment, but from reading the whole verse (Zeph. 2:1-2), we can see that God is warning the people that they have little time to repent. The “Day” is coming and will pass by quickly, like chaff blown by the wind, so if anyone decides to repent they should do it quickly. Once the wrath of God strikes it will be too late.
“O nation without shame.” That the people of Israel could sin egregiously against God and not have feelings of guilt and shame means that they had thoroughly hardened hearts.