Sorry, I cannot find any mention on the General Roman Calendar (US) , however he is noted on the Ruthenian one...
Forgive my mistake St. Maximus...
You won't find him on any liturgical calendar you get from your local Catholic parish because he is not given a universal feast day (in the sense that celebration of it is obligatory), but there are A LOT of Saints who are not there. For instance, the patron saint of my home village in Italy - Santo Stefano del Lupo - is not even listed by name on his feast day of July 19. His entry is: And elsewhere in divers places, many other holy martyrs, confessors, and holy virgins.
If every Saint had his/her own individual feast day that must be celebrated, there would need to be thousands more days in the year.
Here is a link
to the Roman Martyrology for August 13 - Saint Maximus is the fifth entry down:
At Constantinople, St. Maximus, a monk distinguished for learning and for zeal for Catholic truth. Valiantly disputing the Monothelites, he had his hands and tongue torn from him by the heretical emperor Constans, and was banished to Chersonesus, where he breathed his last. At this time, two of his disciples, both named Anastasius, and many others endured divers torments and the hardships of exile.