Here "Ariel" represents the people who live in the city of Ariel. AT: "How terrible it will be for the people of Ariel" (See: [[rc://en/ta/man/translate/figs-metonymy]])
This is another name for Jerusalem, and it means "altar." If possible translate this as "Ariel" rather than "Jerusalem" since the meaning of "Ariel" is important in 29:2. (See: [[rc://en/ta/man/translate/translate-names]])
"Keep celebrating your festival year after year." This is an ironic statement. Yahweh tells the people to keep celebrating their festivals where they sacrifice to him, but he knows it will not prevent him from destroying them. (See: [[rc://en/ta/man/translate/figs-irony]])
The words "mourning" and "lamenting" mean basically the same thing and emphasize the intensity of mourning. AT: "they will mourn deeply" (See: [[rc://en/ta/man/translate/figs-doublet]])