The answer depends on the type of bulbs you are planting and where you live.
These bulbs are planted in mid to late fall. In northern areas, the bulbs should be in the ground several weeks before the soil starts to freeze. For a regional planting map, read: Planning & Selecting Fall-Planted Bulbs.
Spring-blooming bulbs grow roots in the fall, before they enter winter dormancy. To flower properly, these bulbs need to go through at least 12 weeks of consistently cold temperatures (below 45°F). If you live in a warm climate (growing zones 8+) you may need to pre-chill your bulbs before planting. For more information, read How to Grow Spring Bulbs in Warm Climates.
Summer-blooming bulbs include dahlias, lilies, glads, cannas, caladium, elephant ear, begonia, calla lily, anemone, ranunculus, adidanthera, crinum, brodiaea, crocosmia, eremurus, eucomis, hymenocallis, nerine.
These bulbs are planted in spring, once night-time temperatures are above 50°F and there is no danger of frost. In cold climates, you can give these bulbs an early start by planting them in pots, 4 to 6 weeks before transplanting outdoors.
Tropical bulbs such as caladiums, elephant ears, cannas, calla lilies, crinum and eucomis, will not grow until the soil temperature is at least 65-70°F. Planting these bulbs too early may stunt their growth or cause the bulbs to rot. It’s better to wait until the soil is warm (in northern areas this may be mid to late June). Or consider starting the bulbs in pots and transplanting them once the soil has warmed up. For a planting map and additional information, read: Planning for Spring-Planted Bulbs.
Winter-Blooming Bulbs include amaryllis and paperwhites. These bulbs can be planted indoors, anytime from mid-fall through early spring. The bulbs are sensitive to cold and must not be exposed to freezing temperatures. For more information read: All About Amaryllis or All About Paperwhites.