I posted this last year regarding the plenary indulgence available on December 31:
There are two (principal) types of indulgences: partial and plenary. In the vocabulary of the Church, an indulgence removes some (or all) temporal punishment for sin. What that means is that in gaining indulgences, we can shorten (partial) or eliminate (plenary) the consequences of sin (i.e., time in Purgatory) for ourselves, or, for the souls already in Purgatory.
Through original sin, and in our own lives in which we commit various sins, our soul suffers under the weight of sin, but the ultimate punishment (death and separation from God) is no more, because we are saved by Jesus Christ. However, just because God forgives sin does not mean that He removes all of the consequences of sin for us. Just like the lad who hits the baseball through his neighbor’s window, we must “pay to repair the window” — we must face the consequences of our sin.
God is infinitely merciful, so that even His “consequences” are especially helpful and loving to us. Purgatory exists so that we might “work out” the consequences of sin and its stain on our souls before entering into heavenly beatitude with God. It is a promise that even if we fail to allow ourselves to be perfected in this life, we need not worry about work yet to be done, because God will help us get there even after death. Purgatory is the fire that “purges” all that remains as separation between us and God. It is the final Clorox cycle on our souls. It purifies and whitens!
And, don’t forget that Purgatory is not a proving ground. It is not a place where God decides whether we are saved; rather, our salvation is already guaranteed — we’re on the way to Heaven!
However, souls in Purgatory cannot pray for themselves, which is why it is a spiritual work of mercy to pray for the souls there.
On the last day of the year, you can gain a Plenary Indulgence (the best kind!) if you recite the Te Deum, an ancient hymn of the Church:
O God, we praise Thee, and acknowledge Thee to be the supreme Lord.
Everlasting Father, all the earth worships Thee.
All the Angels, the heavens and all angelic powers,
All the Cherubim and Seraphim, continuously cry to Thee:
Holy, Holy, Holy, Lord God of Hosts!
Heaven and earth are full of the Majesty of Thy glory.
The glorious choir of the Apostles,
The wonderful company of Prophets,
The white-robed army of Martyrs, praise Thee.
Holy Church throughout the world acknowledges Thee:
The Father of infinite Majesty;
Thy adorable, true and only Son;
Also the Holy Spirit, the Comforter.
O Christ, Thou art the King of glory!
Thou art the everlasting Son of the Father.
When Thou tookest it upon Thyself to deliver man,
Thou didst not disdain the Virgin’s womb.
Having overcome the sting of death, Thou opened the Kingdom of Heaven to all
Thou sitest at the right hand of God in the glory of the Father.
We believe that Thou willst come to be our Judge.
We, therefore, beg Thee to help Thy servants whom Thou hast redeemed with Thy
Let them be numbered with Thy Saints in everlasting glory.
V. Save Thy people, O Lord, and bless Thy inheritance!
R. Govern them, and raise them up forever.
V. Every day we thank Thee.
R. And we praise Thy Name forever, yes, forever and ever.
V. O Lord, deign to keep us from sin this day.
R. Have mercy on us, O Lord, have mercy on us.
V. Let Thy mercy, O Lord, be upon us, for we have hoped in Thee.
R. O Lord, in Thee I have put my trust; let me never be put to shame.