18 “This is the day which the Lord has made”. This man remembers that he had said in former Psalms, “Since He has inclined His ear unto me, therefore will I call upon Him as long as I live;” making mention of his old days; whence he now says, “This is the day which the Lord has made;” that is, wherein He has given me Salvation. This is the day whereof He said, “In an acceptable time have I heard you, and in a day of Salvation have I helped you;” that is, a day wherein He, the Mediator, has become the head Stone of the corner. “Let us rejoice,” therefore, “and be glad in Him.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)