This is very much old school Roman Catholic. It is a Lenten hymn with a text by St Gregory the Great translated by Ronald Knox and set to SONG 5 by Orlando Gibbons.
I got the chords from Liturgyshare for a backing:
1 Keep we the fast that men of old
Learned from on high in mystic ways,
Till yonder sun hath duly told his hallowed
tale of forty days.2 This covenant, long since revealed
To patriarchs and ardent seers,
Christ by His own example sealed,
Author of time, and Lord of years.3 More wisely therefore let us walk,
Sparing food and wine and sleep;
Over trifles and our talk more jealous
be the watch we keep.4 Still by our sins, O Lord, we grieve thy love,
So full of pardon free: Author of mercy,
Still reprieve the souls that turn again to thee.5 Remember whence our fashion came,
Frail creatures; yet thy creatures still,
Crush for the glory of thy name,
the murmurings of our stubborn will.6 The guilt that dooms us put away,
With larger grace our prayers requite;
At last, and ever from this day,
teach us to live as in thy sight.7 Hear us, O Trinity sublime, and undivided Unity;
So let this consecrated time bring forth
thy fruits abundantly.
The newer tranlation by Peter Scagnelli, “Again We Keep This Solemn Fast”, is much better known these days.