For the more linear-minded readers: I'm talking about two different things here, intermingled. I'm a person who enjoys a certain amount of data, and working numbers, etc., but I also enjoy feeling the soil beneath my bare feet, and watching for signs of renewed life each spring. Don't lose the forest for the trees.
There are now roughly seven weeks until the frost-free date I go by, April 20. But let me back up. When I moved to New Jersey after six years in Michigan, I was excited to be in zone 7. Some people said I wasn’t, as the USDA Hardiness Zones hadn’t yet been updated, and if a chart says something, well, the chart must be right forever. Chart bedamned; it was easy to tell right off the bat how things would be. This meant for me mainly that rosemary would live through a winter outside. Now the newer zone guide from 2012 calls the areas I lived in 7(a,) because no matter how you wish to view the world, it's all warmer than it was when the old data was used. To be honest, I already grew everything in Mid-Michigan as though it was zone 6, not 5, and other gardeners there did the same. You don't need a chart to tell you everything is growing for nearly seven months instead of less than six, and that some of the plants aren't dying under the winter snow.*
Now I’m back in zone 6. For refined delineations, just west of me, it’s 6b; the urban heat island of Cincinnati, similar to the areas I lived in NJ, except a little cooler in winter. Here a mile or two east, they call it 6a. This actually means little to me other than not expecting rosemary to last in the ground all winter, so I pot it and bring it in. It's very rarely below zero, but the cold we do get is sustained longer.* And most outdoor planting starts about two weeks later. My in-ground herbs perform the same each year, lasting much farther into the year than I’m told to expect, coming back earlier in the spring in the same manner, but I don’t plant tender annuals earlier because of that, for a couple reasons.
First, the soil is rarely ready to be worked until at least the second week of April. April is so agonizing! I stick a thermometer in the ground and watch the soil slooowwwwlllyy rise to above 50  degrees (today, the pots are at 42 [5.5] and the ground is 40 [4.5]) as it also slowly begins turning workable, for putting in carrots, chard, and green beans. This is significantly different from New Jersey, where the soil is very sandy, and warms much faster in spring, though it is not tillable much sooner. The heavier clay-infused soil here is slow to warm, and I grow so impatient waiting for it, I have taken to more and more container gardening each year. I can start a couple weeks earlier that way with some of the things I grow. But that method usually requires more water.
The other reason is that nights here definitely stay cooler for longer into spring, even when the days are very warm. Peppers, in particular, need warm nights in order to grow well. I have three sweet/bell pepper seedlings already going, and planted six from a hot pepper mix and two peperoncinis yesterday, so they might need time in the little plastic greenhouse before beginning life outside sometime in May.
So anyway. Soil temperature and arability, and night air temperatures are my true keys to starting out well in the garden. Based on previous years, I’ve marked my wall calendar with expected tasks I can get done through March and April, and am getting the greenhouse ready for interim housing.
But that’s all data-based stuff,* and what I’m really doing is watching for signs of renewed life outside. Lemon balm appears first, then parsley and mint. I’m hoping to have success with peas this year; never do seem to get many peas, but when the parsley rises, the planting of peas and onions will inaugurate my season of outside pleasures.
PS: Easter dinner in many places has traditionally featured lamb with peas and pearl onions, and mint sauce. Early potatoes with new parsley, perhaps. The tradition is because that’s the fresh stuff available right at the beginning of spring. It’s neat to think about, though I’ve tended to live in areas where an autumn-cured ham was the end of winter holiday tradition instead, on the table with the new stuff just appearing. Isn’t nature awesome?
Another PS: Sometimes I dream of living in zone 8. I’d breathe better in winter. But I’d have to give up the Cincy Symphony, Jungle Jim’s, and the awesome Mt. Washington St. Vincent de Paul for it. Would I be willing to? …well, yeah. Near water; I like the nature of people who are friends with the sea. But that's drifting back in time to another topic altogether.
*For people who want more details on USDA chart drawbacks: Snow insulates the ground and also helps soil renewal, so some very cold places actually have warmer and richer soil than you might imagine, and stuff grows marvelously there in summer. In some places, the temperature range is so extreme, what grows well can't be predicted by how things go in January. To name two drawbacks to a chart based on low annual temperatures.