Objection 1. It would seem that blessing and sanctifying are not due to the seventh day. For it is usual to call a time blessed or holy for that some good thing has happened in it, or some evil been avoided. But whether God works or ceases from work nothing accrues to Him or is lost to Him. Therefore no special blessing or sanctifying are due to the seventh day.
Objection 2. Further, the Latin "benedictio" [blessing] is derived from "bonitas" [goodness]. But it is the nature of good to spread and communicate itself, as Dionysius says (Div. Nom. iv). The days, therefore, in which God produced creatures deserved a blessing rather than the day on which He ceased producing them.
Objection 3. Further, over each creature a blessing was pronounced, as upon each work it was said, "God saw that it was good." Therefore it was not necessary that after all had been produced, the seventh day should be blessed.
On the contrary, It is written (Genesis 2:3), "God blessed the seventh day and sanctified it, because in it He had rested from all His work."
I answer that, As said above (Article 2), God's rest on the seventh day is understood in two ways.
First, in that He ceased from producing new works, though He still preserves and provides for the creatures He has made.
Secondly, in that after all His works He rested in Himself. According to the first meaning, then, a blessing befits the seventh day, since, as we explained (72, ad 4), the blessing referred to the increase by multiplication; for which reason God said to the creatures which He blessed: "Increase and multiply." Now, this increase is effected through God's Providence over His creatures, securing the generation of like from like. And according to the second meaning, it is right that the seventh day should have been sanctified, since the special sanctification of every creature consists in resting in God. For this reason things dedicated to God are said to be sanctified.
Reply to Objection 1. The seventh day is said to be sanctified not because anything can accrue to God, or be taken from Him, but because something is added to creatures by their multiplying, and by their resting in God.
Reply to Objection 2. In the first six days creatures were produced in their first causes, but after being thus produced, they are multiplied and preserved, and this work also belongs to the Divine goodness. And the perfection of this goodness is made most clear by the knowledge that in it alone God finds His own rest, and we may find ours in its fruition.
Reply to Objection 3. The good mentioned in the works of each day belongs to the first institution of nature; but the blessing attached to the seventh day, to its propagation.
Source: Summa Theologica (New Advent)