Holy Week is the seven days this rule counts backward from Easter, beginning with Palm Sunday and closing with Holy Saturday, the church's annual commemoration of Christ's final week culminating in the Passion. The earliest detailed record of the week being kept as a structured sequence of distinct daily observances, rather than simply as ordinary days leading up to Easter, comes from the pilgrim Egeria's account of fourth century Jerusalem, which describes separate station liturgies held at different holy sites around the city on nearly every day of the week, a level of daily structure that later spread outward and shaped how the rest of the Christian world came to observe the week.
Several days within Holy Week already carry their own distinct commemorations, Palm Sunday's entry into Jerusalem, Holy Thursday's Last Supper, and Good Friday's crucifixion among them, so Holy Week as a whole functions as the frame those individual days sit inside rather than a single undifferentiated observance. The week is kept, in some form, by nearly every Christian tradition that observes a liturgical calendar at all, Catholic, Orthodox and many Protestant churches alike, though the specific ceremonies attached to each day, and how elaborately they are marked, vary considerably by tradition, with Orthodox practice in particular retaining an unusually dense sequence of services across the week that echoes the ancient Jerusalem pattern Egeria described.