Maundy Thursday April 22 inclusive, but these dates fall on different days depending on whether the Gregorian calendar or Julian calendar is used liturgically. Eastern churches generally use the Julian calendar and celebrate this feast throughout the 21st century between April 1 and May 5 in the more commonly used Gregorian calendar. The liturgy held on the evening of Maundy Thursday initiates the Easter Triduum, the period which commemorates the passion, death, and resurrection of Jesus; this period includes Good Friday and Holy Saturday, and ends on the evening of Easter. The Mass of the Lord's Supper or service of worship is
Perennial calendar year after year. Advantages claimed for a perennial over an annualized calendar like the Gregorian are simplicity and regularity. Scheduling is simplified for institutions and industries with extended production cycles. School terms and breaks, for example, can fall annually on the same dates. Month-based ordinal dating ("Fourth Thursday in November", "Last Monday in May") will be obsolete. Two methods favored for perennializing the calendar are the introduction of so-called "blank days," and of a periodic "leap week." Blank-day calendars remove a day or two (the latter for leap years) from the weekday cycle, resulting in a year length of 364