Leviticus 22:3
Say unto them, Whosoever he be of all your seed among your generations, that goeth unto the holy things, which the children of Israel hallow unto the LORD, having his uncleanness upon him, that soul shall be cut off from my presence: I am the LORD.
Whoever among all the seed in their generations draws near to the holy things which the children of Israel set apart to the Lord while his uncleanness is upon him shall be cut off from before the divine presence. The severity of that cutting off lays bare how serious sin is and how necessary it is to remain in living connection with God rather than approaching Him while defiled. The book as a whole sets forth the sanctuary services as pointing to Christ, with the final judgment making the ultimate separation between those joined to God and those identified with sin.
The same requirement of holiness in approaching God shows that the Jews were to gather in holy convocation for their spiritual prosperity and to be instructed in the testimonies concerning a Saviour to come, so that public worship formed an important part of Sabbath observance. That need continues undiminished for Christians to assemble on the Sabbath to celebrate the fulfillment of those testimonies and the grace and truth that came by Jesus Christ, since public worship promotes growth in holiness and belongs together with the Sabbath wherever circumstances do not make it impossible.