16 The preterite with the vav (ו) consecutive is subordinated as a temporal clause to the next preterite.
17 The word “this” has been supplied.
18 “Before it” means before the deity in the form of the calf. Aaron tried to redirect their worship to Yahweh, but the people had already broken down the barrier and were beyond control (U. Cassuto, Exodus, ).
19 Heb “called.”
20 The word is חַג (khag), the pilgrim’s festival. This was the word used by Moses for their pilgrimage into the wilderness. Aaron seems here to be trying to do what Moses had intended they do, make a feast to Yahweh at Sinai, but his efforts will not compete with the idol. As B. Jacob says, Aaron saw all this happening and tried to rescue the true belief (Exodus, 941).

In other versions

When Aaron saw this, he built an altar before it. And Aaron made a proclamation and said, “Tomorrow shall be a feast to the Lord.”

When Aaron saw this, he built an altar in front of the calf and announced, “Tomorrow there will be a festival to the Lord.”

When Aaron saw this, he built an altar in front of it and made an announcement: “There will be a festival to the Lord tomorrow.”

Aaron saw how excited the people were, so he built an altar in front of the calf. Then he announced, “Tomorrow will be a festival to the Lord!”

And Aaron saw, and he built an altar before it, and Aaron called, and he said, “A feast for Yahweh tomorrow.”

And when Aaron saw it, he built an altar before it; and Aaron made proclamation, and said, To morrow is a feast to the Lord.

Public Domain