Act of Contrition, (the simple truth)
​ Lord Jesus, Son of God, have mercy on me, a sinner.

Or...
O my God, I am heartily sorry for having offended Thee, and I detest all my sins because of thy just punishments, but most of all because they offend Thee, my God, who art all good and deserving of all my love. I firmly resolve with the help of Thy grace to sin no more and to avoid the near occasion of sin. Amen.

30 - 45 Minutes before
all Daily and Weekend Eucharistic Celebrations

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Before celebrating the Sacrament of Penance, one should prepare themselves with an examination of conscience. 

An examination of conscience is a “prayerful self-reflection on our words and deeds in the light of the Gospel to determine how we may have sinned against God” (— Glossary, The Catechism of the Catholic Church.
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There are various types of examinations of conscience but regardless of which one you use to prepare yourself for the Sacrament it should be rooted in Scripture; particularly, the Ten Commandments and Beatitudes.  Below are a few examples of Examinations of Conscience that can help you prepare for the Sacrament.

Examination of Conscience based on the Ten Commandments
Examination of Conscience in Light of Catholic Social Teaching I En Espanol
Examination of Conscience for Children
Examination of Conscience for Young Adults
Examination of Conscience for Single People
Examination of Conscience for Married Persons

Other Resources are available on website of the United States Conference of Catholic Bishops

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