Holy Mother Church offers the possibility to gain two plenary indulgences, one on 31 December to thank God for the graces received during the year and one on 1 January to invoke the Holy Spirit at the beginning of the new calendar year.
Enchiridion Indulgentiarum, #26
A plenary indulgence is granted to the Christian faithful who, in a church or in an oratory, are present in a recitation or solemn chant of:
1° the hymn Veni Creator … on the first day of the year, imploring divine assistance for the whole of the coming year…
2° the Te Deum hymn, on the last day of the year, in thanksgiving to God for the favors received in the course of the entire year.
Conditions for a Plenary Indulgence from the Apostolic Penitentiary, Prot. N. 39/05/I
In addition to the indulgenced act or “good work” (recitation or solemn chant of the hymn Veni Creator/Te Deum hymn on the due day) the usual following conditions must be observed for those who want to gain a plenary indulgence:
1. State of grace at least when performing the indulgenced act;
2. Complete detachment from sin, even venial sin;
3. Confession (within 20 days before or after the indulgenced act);
4. Communion (within 20 days before or after the indulgenced act);
5. Prayer for the intentions of the Supreme Pontiff (left to the choice of the faithful, e.g. Our Father & Creed).
A plenary indulgence can be gained only once a day. One sacramental confession suffices for several plenary indulgences, but a separate Holy Communion and a separate prayer for the Holy Father’s intentions are required for each plenary indulgence.
What is an indulgence?
An indulgence is the remission before God of the temporal punishment remaining for sins whose guilt has already been pardoned. It is granted to a properly disposed Christian who fulfills the conditions established by the Church. As the minister of redemption, the Church exercises its authority to dispense the spiritual treasury of Christ and the saints. Any Catholic in the state of grace and not under canonical penalty may gain an indulgence through the Church’s provision.
Indulgences rest upon the doctrine of the communion of saints: all who are united to Christ in grace—those in heaven, the souls being purified in purgatory, and the faithful on earth—share a supernatural solidarity. The merits of Christ, together with the superabundant merits of the saints, form a common spiritual treasury. From this treasury the Church, particularly through the Pope and the bishops, applies what is needed to remit the temporal effects of sin.
What does this mean? 2. Complete detachment from sin, even venial sin;
I wrote about this HERE
As a bonus, waaaaay back in 2006 my good friend Fr. Tim Finigan had a clear explanation of being detached from sin and the disposition you need to gain indulgences. HERE