Feast: Octave of Pentecostes
The well-known Trinity office is ascribed to bishop Stephanus of Luik, early 10th century. Due to the fact that Trinity was celebrated everywhere, the office spread quickly over the entire Christian world.
Though the feast of Trinity was not established before the 14th century, it was widely celebrated before that time.
Auda: Étienne de Liège, pp. 67-121 (edition: pp. 113-121)
Mentioned in Hughes: LMLO, referring to Auda.