Find out the frost-free date for your area from your local cooperative extension office, a garden center, or an experienced gardening friend. Depending on the cold tolerance of the flowers you intend to plant, use this date as a guideline. Cool-season annuals such as pansies, snapdragons, and pot marigolds can be set out several weeks before the last frost date. Tropicals that are extremely sensitive to cold, such as impatiens and Thai basil, should wait until a couple of weeks after the frost-free date.
Keep an eye on the weather; some years the weather warms up much earlier than in others. As a safeguard, be prepared to cover your flowers should an unseasonably late frost occur. Remember, too, that the soil needs to warm up for annuals to grow well. If you have raised beds -- which allow the soil to warm faster than soil of in-ground beds -- you'll be able to plant earlier.