If you wanted to keep a saint in the Canon but hide him from view, February 29 is a good day to assign his feast. After all, it only rolls around once every four years. True, in Marseilles, John Cassian's feast is July 23, but the East celebrates him on Leap Day, and the rest of the West doesn't celebrate him at all.
This sort of oblique suppression suggests some dark, troubled history, doesn't it? Something involving demonic compacts, betrayals, illegitimate children, and maybe even a Saracen or two. Sadly, the questions raised by his career are a lot more intellectual and less sensational.
John Cassian was a fourth century Scythian (Romanian) who moved to Egypt and studied monasticism. Egypt was the place for it -- home of the Desert Fathers. He almost got caught up in the Anthropomorphic Controversy, a dust-up about a proto-heretical idea, but instead took off for Constantinople, where he became a deacon for John Chrysostom. [John's a Big Dog, but I haven't written about him yet. Watch for him next September 13.]
When Patriarch John Chrysostom was deposed by the Synod of the Oak, John Cassian joined some of the other tainted clergy in Rome. Pope Innocent I welcomed them; John went off to Marseilles, where he founded a dual monastery (one for men, the other for women) modeled on the communities of the Desert Fathers. The Rule he imposed became a model for Benedict's Rule, which of course was the foundation of western monasticism.
His writings, however, left his reputation somewhat shaky after his death. His work on the Incarnation was a little too Nestorian to be safely orthodox, even if it wasn't purely heretical. Worse, his criticism of Augustine's concept of salvation wound up seeming semi-Pelagian by placing initial emphasis on human will rather than Divine Grace. It's all right if that doesn't make much sense to you. The important part is that he didn't cross the line of heresy, but he kept scraping his toes on the edge of it. After his death, his books were recommended for monks, but no one else was encouraged to read them. It seems fitting then, that February 29 would be his feast, hidden in plain sight, so to speak.