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 It is a Sabbath of solemn rest to you, and you shall afflict yourselves; it is a statute forever.