St. John's is Canada's gold winner for most challenging weather conditions. It is the foggiest and windiest city in Canada, and among the top five for most freezing rain, least sunshine and coolest summers. It does, however, have a relatively mild winter. So, although you may have to deal with mountains of snow, it'll be mild enough to get outside and actually enjoy them.
Summer is the most enjoyable time to visit St. John's. It can be foggy, but temperatures are generally warm (highs around 68 F/20 C) during the day, with nights in the low 50s F/10-11 C. Rain is common throughout the summer months, and fog sweeps in regularly. Early fall is cooler, with highs around 60 F/15-16 C in September. A warm sweater will be useful any time of year.
Remember that if you're on the island of Newfoundland, you will never be more than 62 mi/100 km from the ocean. Many travelers are surprised by how cool the wind feels, even though the temperature is warm. Fog moves in fast and can take you by surprise in any season, though most often in spring and summer. Expect cool temperatures and always, always pack at least light rain gear. Many of the major storms and hurricanes that cross the southern U.S. unleash their fury in Newfoundland before heading out to the Atlantic Ocean. While temperatures there are cooler than the rest of Canada, people from Newfoundland refer to their climate as character-building and invigorating.
Winter can be stormy, though the Atlantic moderates the cold, sometimes turning the snow to rain or sleet. Only hardy travelers who don't mind weather-related inconveniences should visit then.