Vegetables are either:
Cool-season veggies grow best at temperatures averaging 15°F cooler than those needed by warm season types.
Warm-season veggies require both warm soil and high temperatures (with a little cooling at night) to grow steadily and produce crops.
Cool season plants thrive in cool temperatures (40°F to 70°F daytime temperatures) and are somewhat tolerant of light frosts...
Warm season plants thrive in warm temperatures (65°F to 90°F daytime temperatures) and are intolerant of cool temperatures...
Some vegetables can take Wintertime, cold (freezing) weather much better than others, in fact some only reach their full potential (flavour) if they go through a few "light" freezes or a complete Winter (can be over Wintered in the ground) eg
Knowing the answers to the below questions, plus others, will enable you to work out the correct times of year to plant (sow) and harvest...
Q. What vegetables are you considering growing?
Q. What Hardiness zone are you growing in?
Q. What are the average first and last frost dates for where you are?
From your SE profile I see you are located in "Newfoundland and Labrador, Canada", therefore you might find the below article from Newfoundland Botanical Garden of some interest:
The use of "cool climate" in the question and "winwaed" opening comment, "How long is a piece of string?" :-) had me wondering if there was a definitive definition of "cool climate" used in horticultural...
I couldn't find one online (that's not to say there isn't one).
I asked some "experts" on twitter, no one could point me to a definitive definition.
I'm currently left thinking "cool climate" isn't a good term to be used in horticulture, it's too open ended...
Below are a few responses I got on twitter when I asked, "Help, looking for a definitive definition of "cool climate" used in horticultural..."
Well that is pretty subjective (used a lot in the US, where there is such a huge difference across the country).
"Cool Temperate" is the ecological term for the vegetation zone that most of the UK lies in. It is the area of deciduous forest that runs across Europe. To the south is the "Warm Temperate" or Mediterranean zone, and to the north is the "Boreal" or coniferous forest zone. /via Cool Temperate
"Cool season" crops that do best with daytime highs, max out around 60-70°F, with nighttime lows in the 40s-50s°F and can tolerate light frost. /courtesy of University of Illinois Extension Horticultural Educator, Jennifer Schultz Nelson, Ph.D.
While there's no "definitive" standard, our rough definition ranges from just above the frost threshold up to the mid-70s°F /via Missouri Botanical Garden
Köppen classification system probably best to use. UK is Cfb = maritime temperate with average temp > 10°C in summer etc