Maundy Thursday commemorates the Last Supper, that final meal that Jesus Christ had with his disciples before his arrest.
In Tudor times, on Maundy Thursday, the church was prepared for Easter with water and wine being used to wash the altars, and it was traditional for people to go to confession.
It was also customary for the monarch to wash the feet of poor people and to give alms. You can read all about this, and the various other traditions associated with Maundy Thursday, in my article Maundy Thursday.
