If you have started your seeds indoors, you will need to harden off the seedlings before they are ready to remain outdoors full time.  This basically means that you will be weaning them off of the indoor conditions, and preparing them for the outdoors.

Start the process about one week prior to the day that you intend to transplant the seedlings to their permanent container outside.

On the first morning, put the temporary containers (flats, cups, etc.) outside in a shady area close to your home.  A covered patio is ideal.  If you don’t have a covered patio, place the containers under a table, chair, or under the shade of a large plant or tree.    Bring the seedlings back in at night, and any time it becomes very cold, windy, or rainy.   For the next two days, when you take the seedlings outside in the morning, move the containers a little further from the house.

For the next few days, gradually expose the seedlings to more sunlight, moving them out of their covered area into direct sun for half the day (then moving them back to their covered area for the rest of the day, and throughout the night).

By day seven, the seedlings will be strong enough to absorb the sun all day long, and can remain outside permanently to do so.  On day eight, you can safely transplant the seedlings into the containers that will be their permanent homes for the rest of the season.