Yom Kippur Katanיוֹם כִּפּוּר קָטָן

Yom Kippur Katan (Hebrew: יוֹם כִּפּוּר קָטָן) is a minor day of atonement, kept in some communities with a fast or with penitential prayers on the day before Rosh Chodesh. It is not kept before Rosh Chodesh Tishrei, Cheshvan, Tevet or Iyar. When the day falls on Shabbat it moves back to Thursday, and when it falls on Friday many move it to Thursday as well.