Ambrosian Hymn

The Te Deum (also known as Ambrosian Hymn or A Song of the Church) is an early Christian hymn of praise. The title is taken from its opening Latin words, Te Deum laudamus, rendered as "Thee, O God, we praise".

The hymn remains in regular use in the Catholic Church and Methodist Church (mostly before the Homily) in the Office of Readings found in the Liturgy of the Hours, and in thanksgiving to God for a special blessing such as the election of a pope, the consecration of a bishop, the canonization of a saint, a religious profession, the publication of a treaty of peace, a royal coronation, etc. It is sung either after Mass or the Divine Office or as a separate religious ceremony. The hymn also remains in use in the Anglican Communion and some Lutheran Churches in similar settings.

In the traditional office, the Te Deum is sung at the end of Matins on all days when the Gloria is said at Mass; those days are all Sundays outside Advent, Septuagesima, Lent, and Passiontide; on all feasts (except the Triduum) and on all ferias during Eastertide. Before the 1961 reforms of Pope John XXIII, neither the Gloria nor the Te Deum were said on the feast of the Holy Innocents, unless it fell on Sunday, as they were martyred before the death of Christ and therefore could not immediately attain the beatific vision. A plenary indulgence is granted, under the usual conditions, to those who recite it in public on New Year's Eve.

...
Wikipedia