At each Mass, we re-enact Our Lord's giving of the gift of the Eucharist, by doing, in his memory, what he did for us. This "remembrance" makes our celebration a sacrifice and sacrament, allowing us to participate in Jesus' sacrifice upon the Cross. Only at the Last Supper, in giving us the Eucharist and in washing his disciples' feet, did Jesus ask us specifically to do what he did.
In celebrating the Eucharist daily, we do exactly what Jesus did. In imitating his washing the disciples' feet, we interpret his action more generally, as he explained it for us, in terms of service. At evening Mass on Holy Thursday, we often follow Jesus literally in the ceremony of the Washing of the Feet.
On Palm Sunday, without any instruction from Jesus but with a strong sense of participation, we re-enact, more symbolically than realistically, Jesus' entry into Jerusalem by gathering together outside the Church and, with palm branches in our hands, in procession even in only a short distance, into the Church to celebrate the Eucharist.
Of these three re-enactments, only the Eucharist is a sacrament but the other two, Foot-washing, an Entry into Jerusalem, each in its own way, is also an experience of grace.
Lord, as we re-enact Your entry into Jerusalem, be with us in our pilgrimage to the heavenly Jerusalem.