Now the doctrine of the Old Testament was twofold. One was placed openly; the other was veiled under figures and mysteries. The first regarded the unity of God and the creation of the world; the second, the mystery of the Incarnation and reparation. Wherefore, just as they observed the Sabbath as a memorial of creation, so we keep Sunday as a memorial of the Resurrection.
[Thomas Aquinas, Commentary on the Epistle to the Hebrews, Baer, tr. St. Augustine's Press (South Bend, IN: 2006) p. 232.]