2 Wherefore also “on the fifth of the sabbath”? What is this? Let us go back to the first works of God, if perchance we may not there find somewhat in which we may also understand a mystery. For the sabbath is the seventh day, on which “God rested from all His works,” intimating the great mystery of our future resting from all our works. First of the sabbath then is called that first day, which we also call the Lord's day; second of the sabbath, the second day;...and the sabbath itself the seventh day. See ye therefore to whom this Psalm speaks. For it seems to me that it speaks to the baptized. For on the fifth day God from the waters created animals: on the fifth day, that is, on the “fifth of the sabbath,” God said, “Let the waters bring forth creeping things of living souls.” See ye, therefore, you in whom the waters have already brought forth creeping things of living souls. For you belong to the presses, and in you, whom the waters have brought forth, one thing is strained out, another is thrown away. For there are many that live not worthily of the baptism which they have received. For how many that are baptized have chosen rather to be filling the Circus than this Basilica! How many that are baptized are either making booths in the streets, or complaining that they are not made!
But this Psalm, “For the presses,” and “on the fifth of the sabbath,” is sung “unto Asaph.” Asaph was a certain man called by this name, as Idithun, as Core, as other names that we find in the titles of the Psalms: yet the interpretation of this name intimates the mystery of a hidden truth. Asaph, in fact, in Latin is interpreted “congregation.” Therefore, “For the presses, on the fifth of the sabbath,” it is sung “unto Asaph,” that is, for a distinguishing pressure, to the baptized, born again of water, the Psalm is sung to the Lord's congregation. We have read the title on the lintel, and have understood what it means by these “presses.” Now if you please let us see the very house of the composition, that is, the interior of the press. Let us enter, look in, rejoice, fear, desire, avoid. For all these things you are to find in this inward house, that is, in the text of the Psalm itself, when we shall have begun to read, and, with the Lord's help, to speak what He grants us.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)