All the congregation of Israel is identical to verse 3, referring to all the Israelites, or “The whole community of Israel.” Shall keep it, literally “they will do it,” refers to “this festival,” or “this feast” (Revised English Bible), and not to the animal. New Revised Standard Version‘s “shall celebrate it” is an improvement.

Quoted with permission from Osborn, Noel D. and Hatton, Howard A. A Handbook on Exodus. (UBS Helps for Translators). New York: UBS, 1999. For this and other handbooks for translators see here .