Some people plant all their vegetables on the May long weekend. This works for flowers, but it isn’t so great for vegetables.
Cool-weather vegetables go in the garden early April to early May (peas, spinach, lettuce, some brassicas, onions). These plants go in the ground as soon as the soil can be worked, provided you have good drainage.
Mid-weather vegetables go in the garden late April to late May (carrots, potatoes, some brassicas, late lettuces, most herbs). These plants need night-time temperatures steadily above 6 degrees, preferably closer to 10 degrees.
Hot-weather vegetables go in the garden late May to early June (tomatoes, peppers, beans, melons, squash). These plants need night-time temperatures steadily above 10 degrees, preferably closer to 15 degrees. Most of these plants get diseases or stop growing in cool soil.
So instead of looking at the calendar, look at http://www.theweathernetwork.com. Find the 14-day trend. It will show expected day-time highs and night-time lows for the next two weeks.
Since this has been a La Nina spring (cool and wet), spring is late. Don’t plan on putting tomatoes out on schedule this year.
If you’re a new gardener, you might want to consider getting a beginner’s gardening book out of the library and reading it cover to cover. It’s handy to know some basic techniques.