For annuals you will be transplanting from pots, 6-packs or flats bought at the garden center, it's safe to plant when the last frost date is past. For seeds that are expected to sprout in 2 weeks, plant seeds 2 weeks prior to the last frost date.
Old-timers often have rules of thumb for planting times for a given area of the country. For example, in my region (New England, U.S.), gardeners traditionally planted their annuals around Memorial Day (late May). Alternatively, you can find out when the projected last frost date is in your area by checking with your local county extension office.