Nunc dimittis is the Song of Simeon in Luke ii.29-32 (“Lord, now lettest Thou Thy servant depart in peace”.) It is an antiphon, part of the service of Compline in the Roman Catholic Church where it is usually sung in its traditional plainsong or one of the many settings written for it by composers in the styles of their eras.
In the Anglican Church it is sung as part of Evensong, to either Anglican chant or any of the settings written for it during or since the English Reformation. As this service covers the Catholic services of both Vespers and Compline, it usually follows the Marian antiphon, Magnificat. Consequently, many English composers, from Tudor times onward, have written them as a pair.