That is, in LSB, for the Litany:
"The Litany is particularly appropriate in penitential times, whether season (Lent, Advent) or days (Wednesdays, Fridays, and special days of repentance and prayer)." Altar Book, page 410
The assumption imbedded in that rubric is that Wednesday and Fridays are ordinarily days of penitence. This is an old idea among Christians, going back even to the Didache (which may well have been composed before the NT was finished being written):
"But let not your fasts be with the hypocrites; Matthew 6:16 for they fast on the second and fifth day of the week; but do ye fast on the fourth day and the Preparation (Friday)" (Chapter 8)
Thus, rubric 6 links into the great Christian tradition of not merely penitential seasons, but the regular, weekly observance of penitential days. Two days of each week on which to eat less (fast) and to pray for God's mercy.