So how did we get where we are with our planting dates? Historical records show that over the years, May 5th has become that magical date for a good portion of N. Illinois. Live near the Lake Michigan and that date can move to April 30th. Central Illinois is likely to consider April 20 or the 25th their average frost free date. Gardeners waiting a couple more weeks will consider that date the absolute frost free date as every day after May 5, the chance of a frost drops steadily.
In any year, planning when you are going to set out the variety of transplants you have grown or purchased is dependent on what you believe is going to be that average or absolute frost free date. When you decide the date based on the seed packet to start your seedlings indoors for later direct transplanting into the garden or to grow on as a transplant is also based on that magical date.
Gardeners keeping a log can tell you that while you might sow both peppers and tomatoes indoors on the same day, setting those transplants out into garden can differ by two weeks with tomatoes being hardier than those peppers.
We have just passed that average frost free date so those "tender" vegetables like snap beans, summer squash, and sweet corn can be planted from seed. Tomato transplants are a go now too. In a couple more weeks those vegetable plants that really need warmer soils and air temperatures are next up. Those include many of the vine crops like cucumbers and winter squash, okra and lima beans. Transplants that need these same soil and air temperatures include peppers, eggplant and sweet potato slips. Some gardeners will also buy cucumbers as a transplant, getting jump on the first harvests.
If warm season gardening is what you prefer, you can still plant some of the cooler season vegetables as a successive planting. Additional plantings of sweet corn and snap beans are common. Carrots and beets are another consideration.
By planting fresh cabbage transplants in late June you can enjoy cabbage in the fall. Mid July allows a sowing of winter radishes, leaf lettuce or mustard greens. Even in early to mid-August, sowing spinach or spring radishes again will extend your harvest. All of this gets started in early spring based on that May 5th date with plantings or sowings going in ahead of that date and following up with less hardy vegetables and back to hardy vegetables for fall garden.
So however as a gardener you decide when to plant in your yard, that is your magical date!