The answer is Yom Kippur.
Yom Kippur, also known as the Day of Atonement, is the holiest day in Judaism. It is a day dedicated to repentance, prayer, and fasting. Jews seek forgiveness for their sins and aim to make amends with others and with God. The emphasis on repentance is central to the observance of Yom Kippur.
Pesach, or Passover, commemorates the liberation of the Israelites from Egyptian slavery. While it involves reflection and remembrance, the primary focus is on freedom and deliverance rather than repentance.
The Sabbath, or Shabbat, is a weekly day of rest and spiritual enrichment. It is not specifically focused on repentance but rather on rest, family, and spiritual reflection.
Purim is a festive holiday that celebrates the saving of the Jewish people from Haman's plot to destroy them, as recounted in the Book of Esther. It is characterized by joy, feasting, and giving gifts, rather than repentance.