Depending on when you order your seeds, you will have less or more time to start seedlings or plant them directly into the ground. Here's a quick reference for what can be planted when.
Early Seeds: The following seeds can be planted directly into the ground (direct sow), even before the danger of frost has passed: Asian greens, beets, carrots, lettuce, peas, radishes, spinach, turnips.
Post-Frost Seeds: The following should be planted outside after all danger of frost has passed: beans, corn, cucumbers, pumpkins, squashes, Swiss chard.
Indoor Seeds: In warm climates, even tomatoes can be sown directly into the garden. But in cooler zones, it is good to get a jump on the season by starting these vegetables a little early.
Eggplants and Tomatoes: 6 to 8 weeks before last frost
Artichokes: 8 weeks before last frost
Peppers: 8 to 10 weeks before last frost
Broccoli, Brussels Sprouts, Cabbage, and Cauliflower: 4 to 6 weeks before transplanting out
Leeks and Onions: 6 to 8 weeks before transplanting out