If you have a pack of seeds nearby, take a look on the back. Near the lovely descriptions you'll see some numbers. One set should say something about germination and another should give a number of days to maturity. For years as a home gardener, these were just happy little numbers to me. In the best cases they kept me from obsessing too much about how soon I would have those first tomatoes.
Now that I'm a career farmer, these number dictate much of my life. Production farming takes them as gospel truth. There are 52 weeks in the year. Each one is accounted for, and differs by what you're growing, making for a lot to keep track of.
For example, let's say you want to eat a salad with radishes in it. A radish takes a little more than 20 days from seed to ready-to-eat. A standard Salad mix takes about 30 days to grow. So, if you start your salad about a week before your radishes, you should be able to harvest both fresh from the garden at about the same time. A different way of looking at it is if you want that salad May 1st (week 18) you will need to start it the first week of April 2nd (week 14). Seems simple enough. Let's say you want fresh salad every week through early June, when it is traditionally too hot for a quality salad in the mid-Atlantic. You can usually expect to be able to cut salad greens three-to-six times depending on irrigation, sun, and evening temperatures. Yeah! As a result, you won't have to re-plant as much, but you should probably start an extra batch two weeks after the first (week 16), to be safe. On the other hand, radishes are one-and-done. You remove the whole plant when you harvest the radish and you only have about 10 days of peak flavor in the harvest. Better plant radishes every week (weeks 15-18) if you are looking to enjoy them with each of your salad harvests. This is all based on ideal conditions—kind of like scheduling anything before you become a parent. Mother nature can be the ultimate tantrum-throwing toddler.
Back to our salad, let's say average precipitation should be 2" between weeks 14 and 18, but similar to this year, the best you get is an occasional heavy dew. This means more irrigation is necessary. Not a problem if you don't have anything else you are supposed to be planting, transplanting, weeding, or otherwise working on. Lets add more to our salad. Who only wants to eat lettuce and radishes? Everybody loves carrots. Carrots take about 60 days to grow. So, do you still want that salad May 1st (week 18)? You need to make sure those carrots are planted and germinating by late February (week 9). Seem simple?
While you are working on the math for your perfect salad and thinking of other ingredients, don't forget about all your future meals. You'll have to keep track of your planting for those, too. Tomatoes are most often referred to by generation on the farm. Given their root structure, and desire to be planted deep, they are often started in very small containers and transplanted into larger containers before being transplanted into the field. In a large operation you could have one day a week devoted to seeding generation two, while another is devoted to transplanting generation one. By the time the first generation goes into the field, there could be 5 or more generations waiting in line behind the first to take up valuable real estate. Of course this always goes smoothly, since it's taking place at the same time that perfect salad is coming together. ;) If you see a farmer from February through April and he or she sounds like Lewis Carroll's White Rabbit you might now have a better understanding of why.
After years of growing, most of the planting schedule becomes second nature. I've spent every New Years Day starting chili peppers for half a dozen years now. There are some great tools out there to help as well. Johnny's Seeds, a long time farm favorite, has a great little calendar tool (http://www.johnnyseeds.com/e-PDGSeedStart.aspx?source=GFM%201/2011). Enter your frost-free date, traditionally April 15th in RVA, and it gives you a nice guideline. There are much more complex versions available, but this is a good start. As a farm gardener, I used a farmers almanac Calendar for years. I was hoping to have this post up about a month ago, but my planting got off schedule and I had to catch up—such is the life of a farmer.