Epiphany comes from a Greek word meaning "appearance." It was used in Greek literature in many secular ways, so its inclusion in a Greek religious text does not in itself signify the feast day that comes about 12 days after Christmas in the contemporary world. In a Christian text from the second century, according to the Oxford History of Christian Worship, Clement of Alexandria alludes to the Eastern event (but not the name Epiphany) celebrating the baptism of Jesus in the River Jordan:
"The followers of Basilides celebrate the day of his baptism also, spending the night before in reading."[See Clement of Alexandria Stromata 1,21,146.]
Translated by Bainton
"Basilidian Chronology and New Testament Interpretation,"
Roland H. Bainton;
Journal of Biblical Literature , Vol. 42, No. 1/2 (1923), pp. 81-134.
The Oxford History says that in the West, Epiphany refers to the visit of the three wise men (the magi), but in the East refers to the baptism or the birthday.
Bainton says some celebrated the important religious Epiphany as early as the writing of the fourth Gospel. A fourth century pagan writer, Ammianus Marcellinus, is said to be the first extant writer to use Epiphany as the name of the Christian festival.
An early allusion in Christian material and a later named reference from a non-Christian writer don't exactly answer the question. Do you know more? Please post in the comments.