"Maundy" Thursday comes from the Latin mandatum, "commandment." The term refers to the footwashing at the Last Supper when Jesus spoke of the new commandment to love one another as he himself had loved.
Because Good Friday commemorates Christ's death on the cross, people often wonder about the significance of the word "good." It probably derived from the term "God's Friday," just as "good-bye" derives from "God be with ye."
—taken from Seasons and Symbols published by the Augsburg Publishing House