Invitatory

"[1] The invitatory is used to start Nocturns in the Liturgy of the Hours, the Catholic Church's Divine Office.

After the reform of the Liturgy of the Hours following the Second Vatican Council, the Invitatory is said either before the Office of Readings or Lauds, whichever is said first in a liturgical day.

Verse 17 of Psalm 50(51) Domine, labia mea aperies is often used as the invitatory antiphon in the Liturgy of the Hours.

The invitatory may be spoken or sung; there are several musical settings in plainsong or Anglican chant.

[4] An invitatory psalm may also be substituted for the Phos Hilaron in Evening Prayer.