What Is Yom Kippur?

 https://www.chabad.org/library/article_cdo/aid/177886/jewish/What-Is-Yom-Kippur.htm

What: Yom Kippur fast day is the holiest day of the year, when we are closest to G‑d and to the essence of our souls. Yom Kippur means “Day of Atonement,” as the verse states, “For on this day He will forgive you, to purify you, that you be cleansed from all your sins before G‑d.”1

Beyond specific actions, Yom Kippur is dedicated to introspection, prayer and asking G‑d for forgiveness. Even during the breaks between services, it is appropriate to recite Psalms at every available moment.