It’s the night of the last supper, the first mass. It's also the night we commemorate the ordination of the apostles to the priesthood and their commission to “do this in memory of me.” So it’s a special feast day for all priests and a special night to contemplate my vocation. It is the tradition, then, to preach on the priesthood. I am, afterall, a priest for you. Also, there are certainly other men here called to this great vocation, so it is worthy of our meditation.