Mosiah 18: 23

"And he commanded them that they should observe the sabbath day, and keep it holy, and also every day they should give thanks to the Lord their God."

Alma is organizing the church in the forest of Mormon under God's direction.  Here Alma teaches the importance of keeping the sabbath day holy. It is a commandment to do so.  Just as sabbath observance was needed then, it is still needed today.