The Eucharist is the heart and the summit of the Church's life, for in it Christ associates his Church and all her members with his sacrifice of praise and thanksgiving offered once for all on the cross to his Father; by this sacrifice he pours out the graces of salvation on his Body which is the Church.

The Eucharistic celebration always includes: the proclamation of the Word of God; thanksgiving to God the Father for all his benefits, above all the gift of his Son; the consecration of bread and wine; and participation in the liturgical banquet by receiving the Lord's body and blood. These elements constitute one single act of worship.

The Eucharist is the memorial of Christ's Passover, that is, of the work of salvation accomplished by the life, death, and resurrection of Christ, a work made present by the liturgical action.

It is Christ himself, the eternal high priest of the New Covenant who, acting through the ministry of the priests, offers the Eucharistic sacrifice. And it is the same Christ, really present under the species of bread and wine, who is the offering of the Eucharistic sacrifice.

Catechism of the Catholic Church 1407-1410

When do we celebrate the Holy Eucharist?

First, Communion is typically received in the second grade after as part of PSR and after the First Reconciliation has been completed.  Receiving the Sacrament of Holy Communion for the first time as an adult is part of the Rite of Christian Initiation of Adults or RCIA.