This coming Sunday is the first Sunday after Easter and is known by several names.
First, it is called Low Sunday because it stands in contrast to last Sunday, which was the great feast of Easter, the Feast of feasts!
It is also called the Octave Day of Easter, being the 8th day since Easter.
Not primarily liturgically, by mainly popularly, meaning in the common speech of the people, this coming Sunday was also called Quasimodo Sunday, after the first word of the Introit : Quasi modo geniti infantes.
But the primary name for this coming Sunday is Dominica in Albis, or rather Dominica in Albis Depositis, meaning Sunday of the Laying Aside (depositis) of the Baptismal Robes (albis).
You can see in this painting a man, just baptized moments ago, putting on his white baptismal gown in the background, while a man is just getting baptized, kneeling in front.
These baptismal gowns were white, and the Latin word for white is albus. When the adults were baptized at the Easter Vigil, they started to wear these albs or white, baptismal gowns all week during the Octave. On this coming Sunday, they took them off and resumed regular clothing. Thus - Dominica in Albis Depositis.
This is why the Church took for its Introit this Sunday the words Quasi modo geniti infantes, which means "as newborn babies," because the newly-baptized were "born again" as infants in the family of God.
The Gospel tells us the story of the Apostle Thomas, who believed when he saw Jesus in the flesh. Jesus tells him, "Blessed are they that have not seen, and have believed."
In this same Gospel story, the Lord gives the Apostles the power to forgive sin - the Sacrament of Confession!
We were not there to hear the Lord give this power to them. But we believe. Blessed are we, according to the Lord's own words.
In all the Seven Sacraments, faith is needed; faith in what we do not see. We do not see Original Sin depart from the baptized; nor Sanctifying Grace enter or be increased in the person receiving this or that sacrament. When a man is ordained a priest, nothing outward changes about him, but we believe that he is marked forever as a priest, and that will never leave him, even if he should leave the ministry.
Thus, one of the depictions of Low Sunday is the first picture at the top, which shows all the Seven Sacraments. All Seven require faith; to believe what we cannot see with the eyes of the body.
Especially - the Eucharist. We see bread and wine; we believe they have become the true Body and Blood of Jesus, and are no longer bread and wine.
No wonder, from Apostolic tradition, the priest inserts the phrase - MYSTERIUM FIDEI - the Mystery of Faith - during the second and last consecration, that of the wine.