The Catholic Church celebrates the Easter Vigil sometime after sunset on Holy Saturday and before sunrise on Easter Sunday morning. The Easter Vigil, with its four distinct parts, is notably absent from celebrations during Holy Week by our Protestant brothers and sisters.
The Easter Vigil service can last several hours, so perhaps that is why many Catholics do not attend. I find the Easter Vigil to be one of the most beautiful liturgies of the year and a celebration that directly anticipates Divine Mercy Sunday, which follows on the Sunday immediately after Easter.
When possible, the vigil begins outside the church structure with the blessing of both a small bonfire and the brand-new Easter candle. The priest prays for our protection by the “holy and glorious wounds of Christ,” and that the “light of Christ rising in glory [may] dispel the darkness of our hearts and minds.”  The Easter candle is lit from the bonfire, the people light their candles from the Easter candle, and after the prayers of blessing, all process into the church following the lighted Easter candle.
During this first part of the Easter Vigil, the interior of the Church is completely dark with the only light emanating from the large Easter candle and the people’s flickering luminaries. The cantor sings the Easter Proclamation or Exsultet. The words of the Exsultet describe the symbolism of the Easter candle’s light while tracing man's journey from “ancient sinfulness,” starting with the first parents, to the end of “gloom of sin,” through Christ’s death on Good Friday and His anticipated resurrection on Easter Sunday morning.
About half-way through the Exsultet, the cantor chants,
“O truly necessary sin of Adam,”
and then two lines later
“O happy fault.”
“O truly blessed night,
When things of heaven are wed to those of earth,
And divine to human.”
The idea that the sin of our first parents is a happy thing or necessary may sound confusing, at best, and wrong, at worst. Why has the sin of our first parents been recast as something necessary and happy? And why is the Holy Saturday Vigil said to commemorate “when things of heaven are wed to things of earth, and divine to the human?”
I find the answers to these questions in the Divine Mercy of God which is celebrated on Divine Mercy Sunday. Without the knowledge of my sins and the acknowledgment of my brokenness, I have no hope to understand what God offers me in His forgiveness of my sins. When I see my sinfulness, stripped naked of all excuses and illuminated through God the Father’s sacrifice of His only beloved Son and the Son’s complete obedience to the Father’s will, I could find myself in total, abject desolation.
I refuse to take the road to desolation; instead, I choose to view even my sins as a path, paved with God’s unfathomable Divine Mercy, which leads me back to Him. Thus the cantor proclaims “O truly necessary sin of Adam” and “O happy fault.”
God’s Mercy, as exercised in the forgiveness of my sins, and coupled with the knowledge and repentance of my sins, brings me to the intimacy with God for which I was created. All my hope in this life is made possible through Christ’s obedience, death, and resurrection—His glorified wounds—and celebrated uniquely in the Easter Vigil service on Holy Saturday.
Though the transformative grace of Christ’s death and His Easter morning resurrection, all sin can now be sourced into hope. Sin becomes redeemed, resurrected through grace. Our "happy faults" expose our utter dependence on God. And what is God's response? Divine Mercy—God's New Covenant—sealed in the glorified wounds of Christ
How fitting that even in the glorified body of Christ the wounds of His crucifixion are still present. How appropriate that Divine Mercy Sunday follows the first Sunday after Easter!
“Mercy, like every grace, always enters through a wound.” 
My cross will always be a crucifix.
It is this Mercy that is the topic of my first novel to be published in 2019.-----
 For more on the Catholic celebration of the Easter Vigil see: http://www.usccb.org/prayer-and-worship/liturgical-year/easter/commentary-on-easter-proclamation-exsultet.cfm
 For the history and message of Divine Mercy Sunday, see http://www.thedivinemercy.org/message/history/marianconnect.php
 Fr. Peter John Cameron, Editorial, O.P. Magnificat, April 2018, Vol 20, No. 2., pg. 4.