Sforno on Deuteronomy 16:8/ספורנו על דברים ט״ז:ח׳

1וביום השביעי עצרת, on that day the Israelites are to be assembled together in communal worship of the Lord. They will sing the song sung by the Israelites after they had crossed the Sea of Reeds on dry ground. This is why this day has been given a special sanctity.

2ולא תעשה מלאכה, if not for that, the seventh day would not be recognisable as holy at all, just as the seventh day of Sukkot is not called a מקרא קודש, holy convocation.