I get asked this question on nearly a daily basis. And frankly, there is not one easy answer to give. There are just so many variables involved.
You need to realize that nearly all gardening models and methods are an unnatural act! They are an attempt by us humans to control and modify the very design of the natural world. That is, we are trying to lord over nature. Don't believe me? Stop doing any gardening or yard work and see what its natural state reverts to in one year, five years, or longer.
I suppose that the logical starting place for answering this question is to determine the specific germination and cultural requirements for the plant variety you are interested in growing. Learning where various plants originated from is a useful bit of knowledge for a gardener to possess. For example, if you know that watermelons originated in Africa, you can make a very good guess that they require heat to thrive. More information on vegetable origins can be found by clicking here.
You will also need to consider things like your local climate - both reviewing the historical weather information as well as monitoring and understanding the patterns for the current season. This is the "art" side of gardening. Although you can use scientific data to help you form an educated guess when finalizing your plans, you ultimately have to learn to take your queues off of nature and use your gut. It is a learned skill, based on instinct, that agriculturalists have practiced since the beginning and that no one fully masters in a lifetime.
Some useful tools to aide you in planning your planting dates include following the instructions on your seed packets, review your garden logs from past years (You do keep a garden log, right?) reviewing the historical average first and last frost dates for your area, and understanding the optimum germination temperature for the types of seeds that you are sowing. The following link will lead you to a general planting guide table, but if you would like a more practical, portable tool, check out "Clyde's Garden Planner" which we now offer on our main store site.
Raising a successful and productive garden does require study, knowledge, preparation and planning. Start with your garden plot drawing, the list of the things you want to grow, use the information and tools presented above, and decide what you want to plant on what days.
1. The exception would be foraging, nature crafting or even permaculture. All of these methods are useful skills and require less effort on your part for the production part of the food cycle. However, there is a reason that agriculture has evolved over the past several thousand years. By focusing efforts on raising produce, harvests are easier, more predictable, and more abundant than foraging and gathering what nature may produce in any given year.
2. Soil temperatures are very important for successful seed germination. You can purchase very inexpensive temperature probes at your local garden center or hardware store.