The best time depends more on your last frost date than your planting zone.
The planting zone tells you how cold it gets during the winter. The last frost date tells you when tender plants like squash and tomatoes can reliably survive outside.
You can look up your last frost date at this NOAA site. Gourds and other squash family should go out on or after the last frost date. Spinach and kale can go out as soon as the ground is open. Other plants fit somewhere in between, depending on how hardy they are.