The best time to plant flowers depends on the species, but spring and fall are the most common seasons for seeding. Most flowers require a specific climate and soil composition to properly grow and bloom.
The only way to be certain when a flower should be planted is to review that species' preferred hardiness and heat zones - information usually available from the seed packet or garden center where they were purchased. Flowers are separated into annuals, biennials and perennials based on the length of their life cycles. Annuals live for one year. Biennials live for two years, but do not blossom until the second. Perennials last many years and are often more hardy than annuals and biennials.