Maundy Thursday, also known as Holy Thursday, Covenant Thursday, Great and Holy Thursday, Sheer Thursday and Thursday of Mysteries, refers to the Thursday before Easter Sunday.
According to BibleGateway, the consensus definition of "Maundy" means "commandment," and is derived from the Latin word Mandatum, itself from the verb Mandare.
The commandment this word refers to is the one Jesus expressed in John 13:34-35 at the Last Supper to His disciples: "A new command I give you: Love one another. As I have loved you, so you must love one another. By this everyone will know that you are my disciples, if you love one another."
Churches often mark this day with a foot-washing service and a subsequent stripping of the altar of its vestments.