Leo the Great, Pope of Rome (391 or 400–461) (aka Leo I) was born in Tuscany (or Rome), and gained fame as a helper and diplomat while still a deacon. When Pope Sixtus III (not to be confused with the sixty-third pope, who was Pelagius II) died, Leo was unanimously elected to replace him. He ruled the See of Peter during a time when the barbarians of the north (Vandals, Goths, you know the sort) were all Arians, and were generally keen on sacking Rome for reasons of their own. He was notable for having solidified (with a little help from the emperor) the power of the Papacy over the bishops of Gaul (and other places), who had up to that point a level of autonomy they were (perhaps understandably) unwilling to give up. Unfortunately, when he tried to assert hegemony over the Copts, they demurred (if not demurely).
His great Tome (imaginatively titled The Tome of Leo) was (to be honest, somewhat gushingly) approved by the Council of Chalcedon, which accepted his wording, ruling that the human and divine natures of Christ exist “in one person without confusion or division.” In addition to the Tome he left 173 letters and 96 sermons, and we will finish with a quote from his somewhat Chrysostomic Christmas sermon: “There is for all one common measure of joy, because as our Lord the destroyer of sin and death finds none free from charge, so is He come to free us all. Let the saint exult in that he draws near to victory. Let the sinner be glad in that he is invited to pardon. Let the gentile take courage in that he is called to life.”
Colmán of Lindisfarne (ca. 605–675) was bishop of, well, Lindisfarne*, and inventor of the camp stove†. A disciple of the great Columba (Jun 9), he followed Finan (Feb 17) as the abbot of Iona*, as bishop of Lindisfarne, and as champion of the “Celtic” method of calculating the date of Easter.
Oswy (aka Oswiu), king of Bernicia, was raised in the Irish tradition and used the older dating method, whereas his wife had been baptized by Anglo-Saxons and used the “Roman” method. Thus while one of them was celebrating Easter, the other one was still in Lent. Apparently this didn’t bother anybody (enough to say anything) while St. Aidan (Aug 31) was still bishop, but once he died all heck broke loose. The whole thing was so bothersome to Oswy that he called the Council of Whitby* in 664.
There Colmán argued vehemently for the Celtic method, citing Aidan (Aug 31) and Columba (Jun 9) and even John the Beloved Apostle. His (primarily Saxon) opponents did not deny the lineage of the method, but rather pressed the primacy of Peter, and the Pope’s right to fiddle with the dating if he so chose. They also argued that while John was certainly doing the best he knew, the Pope’s fiddling resulted in a superior method. The council (perhaps knowing which side their daily bread was buttered on) ruled in favor of the Saxons, whereupon Colmán gathered up his skirts and his loyal monks (and half of St. Aidan) and fell back to Iona. From there he went to the wonderfully-named Innishboffin, and when bickering broke out between English and Celtic monks there, he founded a monastery in Ireland called “Mayo of the Saxons.”
Alcuin praised the monks for accepting voluntary exile, and for sticking it out among the “very barbarous” Irish (who had been Christian hundreds of years before the Saxons, but hey). Bede*, who holds little truck (and absolutely no passenger vehicles) for the Celts, praised Colmán’s administration of Lindisfarne, citing his frugality, dedication to ministry, and missionary work.
 The more cynical among us might point out that the one thing Colmán was really good at was leaving.