305. When is a person obliged to confess mortal sins?

305. When is a person obliged to confess mortal sins? 

(Comp 305) Each of the faithful who has reached the age of discretion is bound to confess his or her mortal sins at least once a year and always before receiving Holy Communion.

 “In brief”

(CCC 1488) To the eyes of faith no evil is graver than sin and nothing has worse consequences for sinners themselves, for the Church, and for the whole world.

To deepen and explain

(CCC 1457) According to the Church's command, "after having attained the age of discretion, each of the faithful is bound by an obligation faithfully to confess serious sins at least once a year" (Cf.  CIC, Can. 989; Council of Trent (1551): DS 1683; DS 1708). Anyone who is aware of having committed a mortal sin must not receive Holy Communion, even if he experiences deep contrition, without having first received sacramental absolution, unless he has a grave reason for receiving Communion and there is no possibility of going to confession (Cf. Council of Trent (1551): DS 1647; 1661; CIC, can. 916; CCEO, can. 711). Children must go to the sacrament of Penance before receiving Holy Communion for the first time (Cf. CIC, can. 914).     

On reflection

(CCC 2042) The first precept (“You shall attend Mass on Sundays and holy days of obligation and rest from servile labor") requires the faithful to sanctify the day commemorating the Resurrection of the Lord as well as the principal liturgical feasts  honoring the mysteries of the Lord, the Blessed Virgin Mary, and the saints; in the first place, by participating in the Eucharistic celebration, in which the Christian community is gathered, and by resting from those works and activities which could impede such a sanctification of these days (Cf. CIC, cann. 1246-1248; CCEO, cann. 880 § 3, 881 §§ 1, 2, 4). The second precept (“You shall confess your sins at least once a year.") ensures preparation for the Eucharist by the reception of the sacrament of reconciliation, which continues Baptism's work of conversion and forgiveness (Cf. CIC, can. 989; CCEO, can. 719). The third precept (“You shall receive the sacrament of the Eucharist at least during the Easter season.") guarantees as a minimum the reception of the Lord's Body and Blood in connection with the Paschal feasts, the origin and center of the Christian liturgy (Cf. CIC, can. 920; CCEO, cann. 708; 881 § 3). (CCC 1386) Before so great a sacrament, the faithful can only echo humbly and with ardent faith the words of the Centurion: "Domine, non sum dignus ut intres sub tectum meum, sed tantum dic verbo, et sanabitur anima mea" ("Lord, I am not worthy that you should enter under my roof, but only say the word and my soul will be healed.") (Roman Missal, response to the invitation to communion; cf. Mt 8:8) and in the Divine Liturgy of St. John Chrysostom the faithful pray in the same spirit: O Son of God, bring me into communion today with your mystical supper. I shall not tell your enemies the secret, nor kiss you with Judas' kiss. But like the good thief I cry, "Jesus, remember me when you come into your kingdom." 

(Next question: Why can venial sins also be the object of sacramental confession?)