Maundy Thursday or Holy Thursday (also known as Great and Holy Thursday, Holy and Great Thursday, Covenant Thursday, Sheer Thursday, and Thursday of Mysteries, among other names) is the Christian holy day falling on the Thursday before Easter. It commemorates the Washing of the Feet (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 (Spy Wednesday) and followed by Good Friday. “Maundy” comes from the Latin word mandatum, or commandment, reflecting Jesus’ words “I give you a new commandment.” The day comes always between March 19 and April 22, inclusive, and will vary according to whether the Gregorian calendar or the Julian calendar is used. Eastern churches generally use the Julian system.
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. (Source)