Some crops need to mature before the summer temperatures get too high, others don't like the cold at all. Some veggies are especially finicky about being transplanted.
So what's a gardener to do?
First, you need to know when, approximately, you might expect to see the last of the frost. Here in Zone 5/6 that is usually mid to late May.
Adjust this to fit your area, here's a brief Seed Schedule:
12 weeks prior (early to mid March)
Start Indoors all the cole crops: broccoli, brussel sprouts, head cabbage, cauliflower, kohlrabi, collards, mustard and kale.
The long season crops, eggplant, peppers, tomatoes and celery, if you have an outdoor protected area to move them to.
If you have the room: lettuce and spinach.
10 weeks prior or 'as soon as the ground can be worked' (mid to late March)
Direct seed outside: beets, garden and snow peas, fava beans (no other beans), potatoes, onions, radishes, rutabagas and turnips
8 weeks prior(early to mid April)
Direct seed: carrots, lettuce, Chinese cabbage, Swiss chard, spinach
If you started lettuce and spinach inside, transplant out.
Start long season crops, eggplant, peppers, tomatoes and celery, giving them time to harden off before transplanting.
6 weeks prior (mid April-ish)
Transplant your cole crops outside.
Cover the ground where you are going to plant melons with dark plastic and stones or bricks to help warm it up.
2 weeks prior (early to mid May)
If you really feel the need, start melons and watermelon indoors.
I wouldn't recommend it though.
After all danger of frost, you hope (mid to late May)
Direct seed: Southern peas, beans, corn, cucumbers, all squash, melons and watermelon.
Transplant tomatoes, eggplant, peppers, celery.
Consider staggering some direct seeded crops, such as radishes and corn, for a continuous supply; or choose different varieties with various days to maturity.
Since our area was considered Zone 5 and changed to Zone 6, I gave the times for both (earlier times are for Zone 6).
Really though, see how the weather is going before transplanting outside.
Also consider the full moon, when a frost is more likely to occur.
This year it is May 25th., an unlikely but not impossible time for frost. If the weather is warm, I'll push the 'seed packet' and plant earlier.