Impossible to answer such a general question.
When is the right time to plant depends very much on what you are planting. Some things (e.g. garlic and some trees and shrubs) do best planted in fall. Many things do best planted in spring. But there is still a wide variation. Many cold weather veggies (like lettuce, spinach, other greens, broccoli, and others) do best planted in late winter- early spring, more or less a month before your average last frost date. Many others (corn, tomatoes, peppers) can't be planted until after all danger of frost is past. Squash family things are among the last to be planted after the soil is well warmed. There is a similar variation in planting times for various flowers and herbs as well.
AND planting time depends very much on your location and climate. People in the deep south and southwest can really only grow tomatoes from late fall to late spring, when it is cool enough for them. The rest of us grow tomatoes from late spring to early fall.....
I imagine you had some idea that it was an impossible question, so I'm not sure why you asked it that way.
If you look on line you can find planting guides for your area, with appropriate planting times.