Feast of the Three Hierarchs

My St. James Calendar of the Christian Year (one of the best calendars you'll ever buy) says that tomorrow the Eastern Orthodox Church celebrates The Feast of the Three Hierarchs: Saints Basil, Gregory Nazianzus, and John Chrysostom. Sounds like a great feast day, but why is it Chrysostom, instead of Gregory of Nyssa? Nyssa, after all, is always mentioned as the third Cappadocian Father. If anyone can explain why the Father of Mysticism isn't included in this feast, please email me (email link on your left).