Participating in a Catholic retreat or similar program lasting three days or more? A plenary indulgence can be yours!

The main purpose of an indulgence
is to reduce or eliminate
the necessity/duration of a stay in Purgatory, after death.

From the Catechism: An indulgence is a remission before God of the temporal punishment due to sins whose guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions through the action of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satisfactions of Christ and the saints. -CCC 1471

From the Official Manual of Indulgences
NORMS AND GRANTS
Apostolic Penitentiary
UNITED STATES CONFERENCE OF CATHOLIC BISHOPS
Washington, DC

Re: SPIRITUAL EXERCISES AND MONTHLY RECOLLECTIONS

§1 A plenary indulgence is granted to the faithful
who spend at least three entire days in the spiritual exercises of a retreat.

§2 A partial indulgence is granted to the faithful
who take part in a month of recollection.

Subject to “the usual conditions” as specified in the Official Manual:
Make a good confession,
worthily receive holy communion,
pray for the Pope,
accomplish the indulgenced work,
(all) within the specified time period.
If attachment to sin exists, a partial indulgence may be obtained.

There are many other ways of obtaining plenary (complete)
or partial indulgences for yourself and/or for others
who have passed away.

Some examples:

Eucharistic Adoration for at least a half hour (conc. 7 § 1, 1°)
Eucharistic procession (conc. 7 § 1, 3°)
Family, day for consecration of (conc. 1)
First Communion (conc. 8 § 1, 1°)
Good Friday (conc. 13, 1°)
Holy Saturday (conc. 28 § 1)
Holy Thursday (conc. 7 § 1, 2°)
Rosary, Marian (conc. 17 § 1)
Sacred Scripture, reading or listening for at least a half hour (conc. 30)

Much more about Indulgences (including the complete Manual)