The condition of your soils, soil temperature and the date of the average last frost all factor into the determination of when to plant. The plant varieties also come into play, since some can do fine when placed into colder soils, and some can handle light frosts just fine.
Most trees and shrubs can be safely planted as soon as the ground has thawed and has dried enough to crumble when you are digging. If the ground is wet and sticky it is best to wait a bit more so that you avoid compacting the soil. Compacted soil can be a very big challenge to any plant trying to send new roots out to establish itself in your landscape. Even though soil temperatures are cooler than usual for early May, you can go ahead and plant trees and shrubs if the ground has dried enough for good digging.
Cool season annuals such as pansies, snapdragons, alyssum, and violas and cool season veggies can be planted now, again with the caution to do your soil tillage when the soil is dry enough to crumble.
For warm season annual flowers and veggies it is best to wait until the soil temperatures warm up – probably around May 15th – 20th. Most perennial flowers and ornamental grasses should be okay to plant as we near Mother’s Day.