If the feast day of San Juan is near the solstice, then yes, it is St. John the Baptist, because it is celebrated as a holiday (St-Jean Baptiste) in Quebec, and also in Newfoundland. (Apparently St. John's is named as it is because John Cabot landed here on June 24, 1497, and picked the patron saint of the day for the name of the city. That is, however, just hearsay.)