If you’re new to gardening and not sure where to begin one of the most important things to take into consideration is frost/freeze dates. Plants need to be placed in an environment that meets their basic needs, and each of the following is important in determining whether or not plants will flourish: day length, frost, heat, pH, radiation, rainfall, and temperature.
Lower temperatures will trigger a large number of cold hardy plants to go into dormancy. These plants hibernate/sleep through the winter then wake up in the spring. There are, however, many plants that cannot survive lower temperatures.
A great tool to help you determine the best times throughout the year to begin growing plants in your region is the Plant Hardiness Zone Map, which was developed with horticultural and meteorological information in mind, including: changes in weather, classification, interactions with cultural factors (how plants are planted, the size of plants, the way plants are placed in the landscape, etc.), interactions with other environmental factors (humidity, wind, soil type, etc.), introduction of new forms of traditional plants, scope, winter hardiness, and zones.
While there are microclimates within each region and area (areas that may or may not allow you to plant more than gardening books say you can) The Plant Hardiness Zone Map is a great place to begin when planning your garden.
Here is the USDA Plant Hardiness Zone Map via The United States National Arboretum.