Biennial plants flower in their second year of growth, then set seed and die. Say you sow their seeds in the spring of 2012. You will not see blooms until 2013. If you sow the seed in the summer of 2011, however, you should have flowers in 2012. Sowing them in summer gives the plants a chance to establish a strong root system and good top growth before the frost knocks them back. As long as they are hardy in your area, they will come back on strong in spring and bloom that year.
You can purchase seed packets or collect fresh seed from plants already in your garden. Sowing and transplanting them yourself—rather then letting them self-sow—let's you control where the plants show up. This gives you an advantage especially with plants that can self-sow aggressively, such as foxgloves and forget-me-nots. Collect the seed from seed pods when they start to turn brown and dry; be sure to remove the flower stalk entirely once you've collected your seed, to prevent self-sowing.
Sow seeds in a seed tray, leaving several inches between seeds so that they have space to develop several sets of leaves before you transplant them into the garden. (Skipping the usual step of potting them up into individual larger pots.) Alternatively, you might sow them in a row in the vegetable garden, or some other designated "nursery" area, then transplant them wherever you want at the end of the summer. Keeping them separate while they germinate and grow let's you pay special attention to watering and weeding them, and it means they don't have to compete with your established garden plants.
Biennials to sow in summer:
Sweet william (Dianthus barbatus)
Foxgloves (Digitalis spp.—shown above, D. purpurea)
Icelandic poppies (Papaver nudicaule)
Forget-me-nots (Myosotis sylvatica)
Wallflowers (Erysimum spp.)
Need help with propagation? Try reading The Propagator's Bible