Jeremiah 7:29

The people were to cut off their hair as a sign of grief.

"The command to cut off the ’hair’ (lit., ’crown’ . . .) is in the feminine in Hebrew, showing that the city (cf. Jeremiah 6:23 -’O Daughter of Zion’) is meant. The charge stems from the fact that the Nazirite’s hair was the mark of his separation to God (Numbers 6:5). When he was ceremonially defiled, he had to shave his head. So Jerusalem because of her corruption must do likewise. Her mourning is because the Lord has cast her off. Because of her sin, the chief mark of her beauty must be cast away as polluted and no longer consecrated to the Lord." [Note: Feinberg, p. 433.]

They were to go up to a bare hilltop and lament their fate, because the Lord had rejected and forsaken the generation of the Judahites on whom He would pour out His wrath (cf. Jeremiah 7:20).