The Old Testament prophet Joel describes in more detail the day of Judgment. And thus, we are encouraged to prepare for it. Repentance is asked of us before the Day of Reckoning comes. This repentance is shown on the outside by acts of penance, i.e., wearing sackcloth, fasting, and so forth. This consequence of internal sorry for our sins is most natural. Penance should emanate from the interior. Otherwise, it would be fake and contrived.