It is important to harden off seedlings and tender annuals before planting them in your garden or containers.
Gradually exposing them to sun, wind and rain will thicken the cuticle on their leaves so they lose less water. You do not want to shock a seedling. This will cause them to be stunted or die.
You want to start hardening off your plants about 2 weeks before you intend to plant them. Be conscious of last frost date and be prepared to bring your plants inside if temperatures drop. When temperatures are consistently above 45* you can place your seedlings or tender annuals outside. Choose a location away from direct sun and wind. Start with about an hour a day and gradually increase exposure daily. After a few days in a shady area you can increase the amount of time in direct sun.
Once temperatures remain above 50* overnight you can start leaving them out 24 hours. After about 14 days your plants will be ready to be planted!