Albert Barnes Commentary Leviticus 23:3

"Six days shall work be done: but on the seventh day is a sabbath of solemn rest, a holy convocation; ye shall do no manner of work: it is a sabbath unto Jehovah in all your dwellings." — Leviticus 23:3 (ASV)

The seventh day had been consecrated as the Sabbath of Yahweh, symbolizing 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.