The first and last day of Passover/Unleavened Bread were days when no work was done (i.e. Sabbath), also there was the weekly Sabbath.
This is the insanity of strict sola scriptura revealed: the bible wasn’t written with a glossary; the very meaning of words comes from outside the bible. You want to decide that the Sabbath means something other than the seventh day of the week, so you just assert it does. And every little detail becomes a bone of exhaustive contention.