Charles Ellicott Commentary Exodus 32:5

"And when Aaron saw [this], he built an altar before it; and Aaron made proclamation, and said, To-morrow shall be a feast to Jehovah." — Exodus 32:5 (ASV)

Aaron ... built an altar before it. —Having once yielded to the popular cry, Aaron was carried on from one compliance to another. He caused the mold to be made for the idol, and the gold to be melted and run into it; and now he constructed, perhaps with his own hands, an altar of rough stones or turf (Exodus 20:24–25), and placed it directly in front of the Image, thus encouraging the offering of sacrifice to it.

Perhaps he flattered himself that by heading the movement he could control it, and hinder it from becoming downright apostasy from Jehovah. In his view no doubt the calf was an emblem of Jehovah, and the worship paid it was the worship of Jehovah. Hence the festival which he proclaimed was to be a feast to Jehovah. But how little able he was to guide events, or to hinder the worst evils of idolatry from speedily manifesting themselves, appears from Exodus 32:6; Exodus 32:25.