Good Shepherd Sunday occurs on the third or fourth Sunday in the Easter Season. The name derives from the gospel reading for the day, which is taken from the tenth chapter of John's Gospel. In this reading Christ is described as the "Good Shepherd" who lays down his life for his sheep.
In the Traditional (pre-1970) Latin Liturgy (see Tridentine Mass), and in the Book of Common Prayer, this Gospel is set for the 'Second Sunday after Easter' (which is equivalent to the third Sunday of Easter).
In the (Roman) Catholic liturgical calendar and the Revised Common Lectionary this gospel reading is set for the Fourth Sunday of Easter (equivalent to the third Sunday after Easter) and hence some also call this day "Good Shepherd Sunday".
The fourth Sunday of Easter is also kept as Vocations Sunday in many church denominations.