Exodus merely says one day out of seven is a day of rest. It nowhere says that one day has to be a specific day of the week. Which day is the first and which is the seventh of the week is a purely arbitrary construct anyway. Nobody was keeping track from the days of creation to Moses that we know of.
"the Seventh Day" is the name of the day. The Hebrews did not name their days after gods, like the pagans did (still do). They were merely named by their ordinal quality.
The Sabbath is Saturday.