The Sundays following Easter Sunday have various themes, such as Divine Mercy Sunday and Good Shepherd Sunday.
The readings for Good Shepherd Sunday (the Fourth Sunday of Easter, this year falling on April 21) have always included a passage from the Gospel according to St. John in which Jesus refers to himself as the Good Shepherd:
I am the good shepherd. A good shepherd lays down his life for the sheep.
Our Lady, Mother of the Good Shepherd
Historically, the Saturday preceding Good Shepherd Sunday was dedicated to Our Lady, Mother of the Good Shepherd.
This devotion comes to us from Capuchin Franciscans in Spain in the 18th century.
The Capuchins of Central Canada have an extensive history of this devotion on their website (their providence is called Mary, Mother of the Good Shepherd). It all began with a particular friar who depicted Mary in this way:
In the year 1703, Br. Isidore of Sevilla, a great popular preacher, was inspired to be accompanied in his mission by a banner with a particular representation of the Virgin Mary: dressed with the humble clothes of a shepherdess, sitting on a rock, under a tree, wearing an ordinary wide-brimmed hat (sombrero), and with some lambs portrayed around her.
A pious devotion developed around this portrayal of Mary and spread first in Spain and later in Latin America:
[T[he devotion to the "Shepherdess of souls," which popularly became known as the "Divine Shepherdess," expanded rapidly with the creation of many groups of the faithful linked to her throughout Spain. They became known as Mary’s Flock.
Eventually there were even liturgical texts developed for this special feast, used by Capuchins at Mass and in the Liturgy of the Hours.
To this day all Capuchin missions around the world are entrusted to Mary, Mother of the Good Shepherd:
We entrust this great undertaking to the intercession of the Blessed Virgin Mary, Mother of the Good Shepherd, who gave birth to Christ, the light and salvation of all nations, and who, on the morning of Pentecost, overshadowed by the Holy Spirit, presided in prayer at the dawn of evangelization.
While this feast is not celebrated on the universal calendar of the Catholic Church (though some Spanish-speaking countries have various traditions and processions connected to this devotion), the Saturday before Good Shepherd Sunday does not typically have a particular feast on it and a votive Mass in honor of the Blessed Mother could be celebrated then. Texts for the Liturgy of the Hours could also be chosen in honor of Mary.