Ubi Caritas et Amor

The famous Ubi Caritas hymn has been used for centuries as one of the antiphons for the washing of feet on Maundy Thursday. The haunting Gregorian chant melody was composed between the fourth and tenth centuries, though some academics believe the text dates from the earliest Christian times.  

Its theme that God (Deus) is present wherever love (amor) and kindness (caritas) are found makes it an appropriate hymn for all occasions.