|Fresco from the subterranean basilica of Saint Clement in Rome, showing|
Pope Saint Clement (1st century AD) saying Mass. The fresco was likely
done in the 8th or 9th century, though possibly based on much earlier work.
"[Telesphoros] appointed that at the season of the nativity of Our Lord Jesus Christ, masses should be celebrated during the night, for in general no one presumed to celebrate mass before tierce, the hour when Our Lord ascended the cross. And that at the opening of the sacrifice, the angelic hymn should be repeated, namely: 'Gloria in Excelsis Deo' etc. but only upon the night of the Lord's nativity." [Taken from Loomis: Liber Pontificalis (Book of the Popes), p. 12]In her notes, Loomis says that this passage is the earliest mention of midnight Mass at Christmas in the extant literature, though she remains skeptical about the passage's accuracy.
As for the institution of the Gloria, it should be kept in mind that the earliest language of the Roman Church was likely Greek, and that the Liber identifies Telesphoros himself as Greek by birth and an anchorite. Therefore, the Gloria was probably incorporated into the Christmas Liturgy in its Greek form. According to tradition, the hymn was not translated into Latin until the time of Saint Hilary of Poitiers around AD 350. But the provenance is confused. A good attempt at clarification may be found here.
In a further note, Loomis says that the Gloria would later be incorporated into Pontifical Masses, and until the Middle Ages, none but the Pope were permitted to include it.
According to the Liber, and confirmed by Irenaeus and Eusebius, Pope Telesphoros died a martyr during the reign of either Hadrian or Antoninus Pius. His feast day is January 5.
For a related post on the earliest sources for the dating of Christmas, see: