As soon as it is nice (above 10 degrees), you can start putting your seedlings or greenhouse purchases outside. The plants are very tender, or "soft", from being grown in an indoor environment. If you started your own seedling, they may also be getting a little leggy (stretched) from not enough light.
But you can't just put them outside in full sun right away - well you can, but they won't like it! They will burn - just like me (fortunately, I remembered to put sunscreen on yesterday!).
So set them outside in a sheltered location (shade and shelter from too much wind). Start with 30 minutes in shade, increasing to a couple hours over several days. Then start putting them in sun for 10-15 minutes each day, increasing by 15 minutes per day, then back in the shade. The sun and breeze will harden them up. If you have perennials, they will be ready to go outside in a week or so. The traditional date for planting annuals outside is May 24, but you can plant them earlier if you're willing to cover them up or move pots inside anytime there's risk of frost.
Sounds like a lot of work, I know. But it's only for a week or two and your plants will thank you by not going into shock.