The length of the growing season is measured as the time between the last frost in the spring and the first frost in the fall. Whereas the depth of the winter cold limits the perennial plants that will grow in an area, the length of the growing season is the most important climate measure for annual plants.
Vegetable gardens are planted in the spring based on the date of the last spring frost, usually called the “frost-free date” on seed packets. The first frost in the fall kills all the tender plants in the garden, and measures the end of the growing season, at least for tender plants.
For a quick estimate of your local first and last frost dates see Victory Seeds site, Average U.S. First and Last Frost Dates.
For a more accurate local estimate, contact your local Cooperative Extension Service.