Here "Ariel" represents the people who live in the city of Ariel. Alternate translation: "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]])