The Sequence "Veni Sancte Spiritus" has, in my opinion, one of the most beautiful tunes ever written down. The text is attributed to Cardinal Stephen Langton - the thiteenth century Archbishop of Canterbury who famously appears on the Magna Carta. Some have also ascribed the tune to him but, whatever the case, there is certainly a "fit" between words and music.
Before the reform of the liturgy the sequence was said or sung throughout the Octave of Pentecost. One year the choir to which I belonged (in my youth) had the special treat of singing it on three occasions for, in addition to the solemn mass of the Sunday, we had two further high masses for the beginning and ending of the Forty Hours devotion within the octave and for which the mass of the octave took precedence. Happy days!