When is your last frost date?
The PR Farmers Institute suggests end of April. To narrow it down, if you are new to your garden, ask your neighbours. Take into consideration variables like different sides of the house, protective trees, south or north facing slopes, or low-lying spots. Hints from nature may help: buds bursting, early blooms, birdsong and butterflies. Your own record keeping will be the most reliable source.
Once you have a date there is a useful chart from Vegetable Garden Works which helps with the calculations for your planting. It is free to download here
When it comes right down to it the date you use will be a best guess based on the information you have gathered. Once your plan unfold it's time to ramp up your weather observation skills. Cool, clear nights with low humidity, often following a cold front, are signs of an impending frost.
Some things you can do to mitigate damage from a late Spring frost:
Avoid planting tender plants in low-lying areas
Provide wind protectionIf your soil is dry, water plants well the evening before a frost. Moist soil holds up to four times more heat than dry soil. Moist soil also conducts heat to the soil surface faster than dry soil, helping to keep the air above the soil about 5 degrees warmer than when the surface is dry.
Cover plants. Woven fabrics are better insulators then plastics or paper.