Maundy Thursday (also known as Holy Thursday, Covenant Thursday, Great and Holy Thursday, Sheer Thursday, and Thursday of Mysteries, among other names) is the Christian holy day falling on the Thursday before Easter. It commemorates the foot washing (Maundy) and Last Supper of Jesus Christ with the Apostles, as described in the canonical gospels. It is the fifth day of Holy Week, preceded by Holy Wednesday and followed by Good Friday. "Maun-" is from Old Norse, present of munu meaning shall or will; akin to Old English gemynd meaning mind.

The date is always between March 19 and 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 normally celebrated in the evening, when Friday begins according to Jewish tradition, as the Last Supper was held on the feast of Passover, according to the three Synoptic Gospels.

From Wikipedia