By keeping one eye on the signals nature is sending, you can plant your vegetable garden according to nature's calendar.
If you live in the North were crocus are common, you can time your seed sowing to their their bloom: When you see the flowers, plant the following as seeds outside in your garden: broccoli, Brussels sprouts, cabbage, garlic, kale, kohlrabi, onion sets and seeds, peas, potatoes, radish, rutabaga, shallots, spinach, and turnip. Of course these signals don't work if you live where frosts in winter are rare or light, such as in much of coastal California, Phoenix, and along the Gulf Coast and south Florida. In those regions, plant these cool-loving crops in October.
These early plants may be ready to go in while your garden is still cold and wet, before the soil is ready to be worked. One way to tell is to step onto the bare garden soil. If your footprint looks wet and shiny, wait. Another way to test is to squeeze a handful of soil into a ball and set it on ground. Poke it with your finger. If it crumbles, it's okay to plant. If it clings together in a hard ball, wait.
When tulips, daffodils, and maple trees blossom, you can plant beets and Swiss chard by seed. They can take a light frost or two, but not as much as the crops listed above.
When apple trees, lilacs, and late tulips are in bloom, you can plant the following seeds: bush beans, sweet corn, pumpkin, and squash. These crops like summer heat, but can germinate and grow in relatively cool soils.
When the tall, bearded iris bloom and apple blossoms have fallen and summer has arrived, plant seeds outdoors for pole beans, lima beans, cantaloupes, cucumbers, eggplants, peppers, and tomatoes. These are the true heat lovers.