Daylilies are the new millennium's roses. Like roses, they were "discovered" as wildflowers then collected by enthusiasts who crossed and re-crossed them to improve form, color and bloom times. Modern hemerocallis come in a range of sizes and wide variety of flower forms and colors. They also bring a challenge that their common cousins don't present---proper spacing. Consider your perennial daylilies' purpose, size and how soon you want to have to re-set them when spacing them in your garden.
Find the garden center tag or the gardener who gave you the plant to determine how large the plant will grow. Divide that measurement by 2 to find the distance needed between plants to avoid crowding, or its "spread."
Decide how the plant will be used and how soon you want to divide it. Specimen plants should be placed farther from other plants. Add extra space for plants you don't want to have to dig up and re-set in about 5 years, too. If you are planting to propagate new plants or for an instant mass of color that will persist for 3 to 5 years, go ahead and plant each plant so its longest leaves will touch the longest leaves of its neighbors.
Set miniature plants and small daylilies like Stella D'Oro an average of 8 to 12 inches apart from crown to crown. The original Stellas are a bit less compact than newer varieties, but most of them will form a dense mat if set 1 foot to 16 inches apart.
Set medium-sized daylilies from 18 to 32 inches apart. Double-blooms and the showy tetraploids should be set a few inches farther apart so they can get the attention they deserve. Repeat bloomers should be set with extra space, too; you'll need room to get to them to remove spent scapes (the stalks that bear the flowers) to keep them blooming.
Set large daylilies from 32 to 48 inches or more apart. The original cultivar called "Kwanso," a double-petalled tawny lily with an extended period of bloom can spread as wide as 4 feet. Many tetraploid daylilies are bred for flower size and spider daylilies are bred for height; the plants may grow as large as or larger than their common cousin. Kwanso makes an impressive jumble but when it becomes congested, blooming decreases.