Maundy Thursday is the Thursday of Holy Week (the Thursday before Easter). It was the day on which Jesus celebrated the Passover with his disciples, sharing a meal with them which we call the Last Supper.
In those days it was usual for a servant to wash the guests feet on arrival. On this occasion there was no servant present and none of the disciples volunteered to do the menial task. Instead, Jesus got up and washed his disciples feet, giving them an object lesson in humility and service.
In some churches priests carry out a ceremonial washing of the feet of twelve men on Maundy Thursday as a commemoration of Christ's act.
In Britain it is still customary for the sovereign to give 'Maundy Money' to a number of male and female pensioners - one man and one woman for each year of the sovereign's age. The money is contained in two purses: one red and one white. The white purse contains specially minted coins - one for each year of the sovereign's life. The red purse now also contains money, in lieu of gifts which used to be offered to the poor. Up to the time of James II the sovereign also washed the feet of selected poor men.
The word "Maundy" comes from the Latin for 'command' (mandatum). It refers to the command given by Jesus at the Last Supper, that his disciples should love one another.
Thanks to Jeff Osborne for supplying the picture below which shows a bowl with two platforms. The guest would be able to stand on these and have water poured over the feet into the bowl