Early european explorers of North America tried to time visits so that they arrived by Saint John the Baptist's day (late June). This target was both practical (allowed exploration, plus a timely return) and was imbued with good luck. As a consequence, the original settlers named the first place of landing after this Saint. At one point there were three Saint John places in Eastern Canada:
The too-many-Saint-Johns problem was partly behind the renaming business for P.E.I.. If you're travelling in Eastern Canada, don't confuse St. John's with Saint John, unless you want trouble.