Catholics in the Roman Rite celebrate Good Shepherd Sunday during Eastertide.
Similar to "Divine Mercy Sunday," it is fixed on a particular Sunday during the Easter season.
This particular Sunday has always coincided with the following passage from the Gospel according to St. John:
I am the good shepherd. A good shepherd lays down his life for the sheep.
The exact verses change each year, but they all revolve around Jesus calling himself the "Good Shepherd."
Prior to Vatican II this day was celebrated on the 3rd Sunday of Easter, but since then it is always on the 4th Sunday of Easter.
It is a day to focus on Jesus' role as shepherd of souls and our role of following his voice wherever he goes.