Maundy is a special day celebrated by some Christian people, usually around the end of March or early April. It is also known as Holy Thursday or Maundy Thursday.
On this day, some Christians remember an important event that is believed to have happened a long time ago. They believe that Jesus (who they think was a really important teacher) washed the feet of his friends to show them how much he loved them and that he was willing to serve them. Washing someone's feet was a really humble thing to do in those days because feet were often very dirty and smelly. It was actually the kind of work that only servants would do.
So, during Maundy, some Christians do a special ceremony where they wash each other's feet. It's not just for fun or because they are dirty, but because they believe it helps them remember what Jesus did for his friends. It's also a sign of love and humility towards others.