The seventh day had been consecrated as the Sabbath of Yahweh, figuring His own rest; it was the acknowledged sign of the covenant between God and His people. See the Exodus 20:1-11 notes. As such it properly held its place at the head of the days of holy convocation.
Related Commentaries of Leviticus 23:3
Leviticus 23:3
3 Six days shall work be done: but the seventh day is the sabbath of rest, an holy convocation; ye shall do no work therein: it is the sabbath of the LORD in all your dwellings.