Intractable?

Does anyone know why the Masses for the ferias of Lent have a Tract at Monday, Wednesday, and Friday, but not the other days? It would make more sense to me if it were sung Wednesdays, Fridays, and Saturdays (ie, the more penitential weekdays). But no, it is sung on Monday, and yet not on Saturday. I mean, it's the same Ash Wednesday Tract repeating again and again, so it would be a useful thing to memorize either way, but still...