John Calvin Commentary Psalms 118:24

"This is the day which Jehovah hath made; We will rejoice and be glad in it." — Psalms 118:24 (ASV)

This is the day which Jehovah made. He now speaks of that as a happy and pleasant day, on which he was finally established as king over Israel, and his anointing by Samuel was ratified by this event.

Doubtless, all days were created alike by God. Nevertheless, David, with special distinction, calls that the day of God which, after a long period of darkness, had dawned for the welfare of the Church, because it was marked by a notable event, deserving of being remembered by succeeding generations.

And because the Church had thus emerged from a state of deep obscurity, he exhorts the faithful to gladness and joy. He does so particularly because many still displayed ignorance of God's grace or treated it with contempt, and because others were so fettered by their perverse attachment to Saul that they could hardly be brought to yield allegiance to David.