Where do you get the readings on a weekday?

Friends,

Where do you get the readings on a weekday?

Feb. 5 is the Memorial of Saint Agatha, virgin and martyr

What is the Responsorial Psalm?

The St. Agatha MEMORIAL Resp. Psalm is Ps. 31 ("Refrain: Into your hands, O Lord, I commend my spirit")

But the Resp. Psalm of the DAY is this:

Responsorial Psalm
18:31, 47 and 50, 51
R. (see 47b) Blessed be God my salvation!
God’s way is unerring, the promise of the LORD is fire-tried; he is a shield to all who take refuge in him.
2 .The LORD live! And blessed be my Rock! Extolled be God my savior.
Therefore will I proclaim you, O LORD, among the nations, and I will sing praise to your name.
3. You who gave great victories to your king and showed kindness to your anointed, to David and his posterity forever.

If the readings of the DAY are read, must the Resp. Psalm of the DAY be sung?

If the readings of the MEMORIAL are read, must the Resp. Psalm of the MEMORIAL be sung?

Here is what a priest wrote me, but seems to imply that the readings of the MEMORIAL can only be used on a special Mass.

For a special Mass one may read the special readings of the feast.
Modern liturgists try to insist on the weekday readings but they are not
necessarily correct in doing this.