As a general guideline, you can start hardening off seedlings ten to fourteen days before the last frost date. While this date varies from one region to another, fortunately, it's easy to find out by consulting publications such as A Gardener's Journal, The Old Farmer's Almanac, or seed catalogs. You can also obtain this information from your local extension service, or by doing a quick search on the Internet. As that date approaches, take notice of your local weather forecasts for a more realistic start date.
When weather conditions are favorable for bringing the seedlings outdoors, choose a partially shaded site near a fence, the side of a house or a hedge to prevent windburn and sunburn. Start exposing the seedlings to the elements for a few hours, and then bring them back indoors.
A drastic change in the weather and temperature can harm the seedlings. If the seedlings are exposed to direct sunlight, wind and lower temperatures too quickly, you may notice signs of distress, such as burning on the leaves or damaged flower buds. Weak plant material may also break in harsh wind.