The Mass of the Last Supper on Holy Thursday re-enacts the washing of the feet of the Apostles by Jesus. Cardinal Jorge Mario Borgoglio of Argentina traditionally celebrated the Mass in a prison, hospital or hospice for the poor. As pope he will continue this tradition. Pope Francis is the first pope to be elected during lent.
Pope Francis is not the first pope to practice this tradition. Popes John XXIII, Paul XI, John Paul II and Benedict XVI all celebrated Holy Thursday Mass with the poor. Pope Francis will be beginning his ministry by following their example. The gesture of washing the feet of disciples is a symbol of the service required to be Christlike.
Pope Francis is also showing his connection to St. Francis, whose ministry was to serve the poor and to live a life of simplicity.