First things first - you need to know in general when the last frost of the season will be. Anybody reading this if you are South of Ashburn Ontario you could push your dates a little earlier and if you are North of Ashburn, Ontario I'd ad a week or two when planting things outside. It's not so bad when you are dealing with frost hardy crops but if you put out sensitive crops and they get hit by frost you can say goodbye to them.
Ashburn, Ontario is kind of on the border of Zone C and D, meaning our last frost date could be anywhere from May 3rd - May 11th (there is always a risk of frost after these dates so keep an eye on your weather network, this is just a guideline).
If you don't want to get too in detail here is a quick guideline:
Frost Hardy Crops (can start planting from April 15-25th)
- Broccoli - Cabbage - Onions - Rhubarb - Spinach - Early Potatoes
- Brussels - Lettuce - Radishes - Peas - Parsnips
- seeds can always be started inside for early crops
Semi-Frost Hardy Crops (can start planting from April 25th - May 10th)
- Beets - Celery - Carrots - Cauliflower - Early Sweet Corn - Late Potatoes
Semi-Frost Tender Crops (May 15th - May 25th)
- Snap beans - Sweet Corn - Tomatoes
- tomatoes should always be started inside 8-10 weeks before planting
Frost Tender Crops (May 25 - June 5th)
- Lima Beans - Eggplant - Pumpkins - Watermelon
- Cucumber - Peppers - Squash
Here are some great books to use as tools but remember that dates are usually off.