After the prayer to the Holy Ghost, the priest goes to the Epistle side of the altar. There the altar boy pours a little water over his fingers. As the priest washes his fingers, he says a Psalm from the Bible that begins with the words, “I will wash my hands.” This part of the Mass is called Lavabo, because lavabo is the Latin word for “I will wash,” and the first word in the Psalm as the priest recites it in Latin.
The Psalm speaks of two things in particular, desire of pardon for sin and God’s glory. Long ago, the priest washed his fingers because they often became soiled as they received the gifts the people brought. Today, the priest continues to wash his fingers, but as a sign that he wishes his soul to be made clean from sin. The people say the Psalm for the same reasons as the priest, to ask pardon of God for their sins and to tell Him that they wish to honor Him.
Taken from The Kingdom of God series The Holy Sacrifice of the Mass by Ellamay Horan. I am not the Author merely the distributor. God Bless BJS!!