The tradition of midnight mass originated around the year 130 AD. Telesphorus leader of the church in Rome ( 7th Pope) established this service of worship at night and included in the mass the singing of Gloria in Excelsis Deo. This was to be done prior to the offering of the Lord’s cup and bread which was being celebrated in recognition of Jesus birth. The song was referred to as the Angel’s Hymn because the song was based on the words said by the angels to the shepherds when they announced Jesus’ birth.
Luke 2
9 And, lo, the angel of the Lord came upon them, and the glory of the Lord shone round about them: and they were sore afraid.
10 And the angel said unto them, Fear not: for, behold, I bring you good tidings of great joy, which shall be to all people.
11 For unto you is born this day in the city of David a Saviour, which is Christ the Lord.
12 And this shall be a sign unto you; Ye shall find the babe wrapped in swaddling clothes, lying in a manger.
13 And suddenly there was with the angel a multitude of the heavenly host praising God, and saying,
14 Glory to God in the highest, and on earth peace, good will toward men.