“Hadad-Rimmon.” This compound word is made from Hadad, a storm and rain god (most modern scholars think Hadad was the same god as the Canaanite god Baal), and Rimmon, a thunder god (2 Kings 5:18. Some scholars believe Rimmon was also identified with Baal).
The use of the word here in the genitive case, “the mourning of Hadad-Rimmon,” is unclear. It could refer to “the mourning for Hadad-Rimmon” (cp. ESV, NAB, NLT), that is, the mourning done by the pagans when their gods were destroyed by Yahweh.
Also, however, Hadad-Rimmon could be a place name, in which case the phrase would mean “the mourning done at Hadad-Rimmon.” Although there is no known mourning event at a place by that name, the fact that Zechariah 12:11 says, “Hadad-Rimmon in the valley of Megiddo,” has led many to believe that it is likely that Judah mourned the death of the righteous king Josiah at that place in the valley of Megiddo after Josiah was mortally wounded by an arrow (2 Chron. 35:22-25). The fact that it is the families of the Judeans who are mourning and not the pagans (Zech. 12:12-14), tends to support this latter interpretation.