The identification of the author of Breviarium Rerum Gestarum Populi Romani (“Summary of Roman History”) remains controversial, though most scholars now identify him with one Festus of Tridentum, a proconsul of Asia who appears in the work of Ammianus Marcellinus. The Breviarium was probably written during 369-370. A recent synopsis and discussion of Festus’ identity appears in Kelly 2010.
Gavin Kelly, “The Roman World of Festus’ Breviarium” in Unclassical Traditions 1: Alternatives to the Classical Past in Late Antiquity, edd. C. Kelly, R. Flower and M. S. Williams (Cambridge, 2010), 72-89.