Two quotations concerning the hymn.

(1) "You have to get up early if you want to enjoy one of Oxford's most attractive old customs, the May Morning ceremony at Magdalen College.
"People gather very early on the morning of May Day on Magdalen Bridge. At 6:00 a.m., from high above on top of the tower of Magdalen College, the strains of a hymn in Latin float down, sung by the college choir. The hymn is followed by a madrigal and then the morris dancers dance in Radcliffe Square and Broad Street. How this custom started is not known, but it was going strong by about 1650 when the choir sang at 4:00 a.m. Later, musical instruments played and the whole concert went on for two hours, to be drastically reduced one very wet May morning late in the eighteenth century!
"A lot of pubs have a special licence and open for this event. Many also serve breakfast although booking ahead is often a necessity." (From HERE)

(2) "On May 1st of 1915, VB [Vera Brittain] describes the Oxford May Morning ceremony. 'I was up at 3:45 this morning for the famous Oxford May Morning ceremony. Magdalen Bridge was quickly crowded with townspeople, women students, the remaining undergraduates, and bicycles, while the river below was covered with punts and sculling boats. When the clock struck five everyone became suddenly silent, and then, just as the sun was rising, the Magdalen choir from the top of Magdalen tower sang the May Morning hymn. As they sang the sun gradually rose higher and threw a golden glow over the slender grey tower. The voices of the singers sounded very pure and sweet in the clear morning air. I had a choked feeling in my throat...'" (From HERE)

Thus, it is subtitled "The Magdalen Tower Hymn" in Songs of Praise.

~Masato