Rabbi Yosef Yitzchak of Lubavitch once went in to his father (Rabbi Shalom Ber of Lubavitch) after Yom Kippur and asked “What now?” His father answered, “Now, especially, we must repent.”

From the great loftiness of Yom Kippur, when every Jew is on an extremely high level, repentance is demanded of us also in connection with such matters that previously were not considered sin at all, and therefore on the day after Yom Kippur “We must especially repent.”

(Sichat Shabbat Parshat Ha’azinu 5742)