The great thanksgiving of the Church, sung when something has gone right.
An ancient hymn of praise sung at the end of the Office of Readings on Sundays and feasts, and at any public thanksgiving.
When: On the last night of the year, at the end of a council, after a deliverance, or on any day a household knows God has helped it through.
Why: Gratitude that is only felt evaporates. This is gratitude given words the whole Church already agrees on, so it survives the mood that produced it.