This hymn is used for Vespers on Saturdays (I Vespers Sunday) throughout the year in the Extraordinary Form. It is used for Vespers on Sundays of Weeks II & IV during Ordinary Time in the Ordinary Form. This hymn is also used for Vespers on Trinity Sunday.
Now doth the fiery sun decline: —
Thou, Unity Eternal! shine;
Thou, Trinity, thy blessings pour,
And make our hearts with love run o’er.
Thee in the hymns of morn we praise;
To Thee our voice at eve we raise;
Oh, grant us, with thy Saints on high,
Thee through all time to glorify.
Praise to the Father, with the Son,
And Holy Spirit, Three in One;
As ever was in ages past,
And shall be so while ages last.
Words: Attr. St. Ambrose*, 4th C.; tr. Fr. Edward Caswall, 1849.
Tune: “O Lux Beata Trinitas” Gregorian Chant, Tone VIII, traditional.
*The Liber Hymnarius records an anonymous authorship dated to the 7th or 8th C.
The original Latin text may be found here.