St. Thomas Aquinas writes, “[R]est is taken in two senses, in one sense meaning a cessation from work, in the other, the satisfying of desire. Now, in either sense God is said to have rested on the seventh day.” (ST I.I., Q 73, A. 2.) Further, “[I]it is right that the seventh day should have been sanctified, since the special sanctification of every creature consists in resting in God.” (ST I.I., Q 73, A. 3.)
Rest in this sense seems akin to enjoyment, “enjoyment” defined not as having fun, but as possessing something. For example when we speak of the freedoms we enjoy in this country, we’re using “enjoy” in that sense.
When you have been pursuing something, and finally attain it, you then proceed to enjoy it. You may also say that you are resting in its attainment, that is, resting from your pursuit of it. An example might be a man pursuing the love of a woman. Once he has proposed marriage to her, and she has accepted, he then comes to a state of both enjoyment and rest, since he now possesses the thing he was pursuing (and she possesses him too, of course). Similarly with regard to an academic degree, or a job, or climbing a mountain: You strive, strive, strive … and then you reach the goal. Your striving ends and you rest in your achievement, enjoying the fruits of your labor.
When we speak of God resting on the seventh day, what do we mean? Obviously he didn’t need physical rest, so it must be the rest of enjoyment: His work was done, and he found it good, and rested and enjoyed what he had made.
What then is the point of resting on Sunday, the Lord’s Day? Is it merely to rest from our temporal labors, namely our jobs? Or just to spend a day focused on God?
Is it merely that God rested, and commands us to rest, and so we rest (though not very strictly in modern days, it seems to me)? But surely we’re not resting in the same sense in which God rested: That we’ve been creating all week, and now we’re happy with what we’ve created, and we spend Sunday enjoying it?
It occurs to me that in the Catholic context, in the light of the Mass, it might mean something more. If rest is the enjoyment of what you have been pursuing, and if Jesus is literally present in the Eucharist, then the Sunday rest means literally enjoying God. We’re to spend the other six days “pursuing” God, so to speak: In praying and spiritual reading, penance and good works, giving of ourselves and denying ourselves (particularly on Fridays). But on the Lord’s Day we receive what we’ve been pursuing. We enjoy God’s literal presence in the Mass and we rest in it for the remainder of the day.