The Church also has designated this Sunday as the World Day of Prayer for Vocations.
The Gospel at Mass is a discourse of Jesus from the Gospel of John.
From John Chapter 10:
"I am the good shepherd. A good shepherd lays down his life for the sheep. A hired man, who is not a shepherd and whose sheep are not his own, sees a wolf coming and leaves the sheep and runs away, and the wolf catches and scatters them. This is because he works for pay and has no concern for the sheep.
"I am the good shepherd, and I know mine and mine know me, just as the Father knows me and I know the Father; and I will lay down my life for the sheep. I have other sheep that do not belong to this fold. These also I must lead, and they will hear my voice, and there will be one flock, one shepherd.
"This is why the Father loves me, because I lay down my life in order to take it up again. No one takes it from me, but I lay it down on my own. I have power to lay it down, and power to take it up again. This command I have received from my Father."
The image above is from here via A Concord Pastor, who also has posted his homily and music for Good Shepherd Sunday.