John Calvin Commentary Exodus 16:5

"And it shall come to pass on the sixth day, that they shall prepare that which they bring in, and it shall be twice as much as they gather daily." — Exodus 16:5 (ASV)

And it shall come to pass. Because the Sabbath is mentioned immediately, some would limit what is said concerning the law to its observance, and interpret it to mean that God was testing whether the people would faithfully observe the rest commanded of them on each seventh day. But there is a weakness in this explanation. The fact is that after God had promised daily supplies to His people, He now adds the exception, namely, that on the sixth day they should collect a double quantity and set aside half of it for the use of the Sabbath. Thus, the seventh day was truly hallowed before the promulgation of the Law, although it is questionable whether it had already been observed by the patriarchs. It seems probable that it was, but I am unwilling to make it a matter of contention.