The people, both Israelites and sojourners, were required not to work. The day is called a Sabbath. The term “rest" shares the same root as the word Sabbath.
Literally, the phrase can read, It is a Sabbath of solemn Sabbath.
This highlights the holy nature of the day.
31 Ce sera pour vous un sabbat, un jour de repos, et vous humilierez vos âmes. C'est une loi perpétuelle.