Plants need to be hardened off for about a week prior to planting. After all danger of frost, you can begin this process.
Day 1-2: Leave your plants outside from morning until evening in a protected area that gets a few hours of morning sun.
Days 3-5: Move the plants to the garden in their containers. Water and leave outside overnight.
Days 5-7: Warm weather permitting, transplant your plants. Make sure to water them!