Catechism of the Catholic Church
2637 Thanksgiving characterizes the prayer of the Church which, in celebrating the Eucharist, reveals and becomes more fully what she is. Indeed, in the work of salvation, Christ sets creation free from sin and death to consecrate it anew and make it return to the Father, for his glory. The thanksgiving of the members of the Body participates in that of their Head.
2638 As in the prayer of petition, every event and need can become an offering of thanksgiving. The letters of St. Paul often begin and end with thanksgiving, and the Lord Jesus is always present in it: "Give thanks in all circumstances; for this is the will of God in Christ Jesus for you"; "Continue steadfastly in prayer, being watchful in it with thanksgiving." 120
English Translation of the Cathechism of the Catholic Church for the United States of America © 1997, United States Catholic Conference, Inc.