Fall planting allows pansies to establish roots in warm soil. They overwinter and come back in spring. They'll have an earlier start and produce more blooms than spring-planted pansies. You'll not only have the bonus of blooms in the fall (and periodically through the winter), you'll get a more spectacular show in spring. If you plant in fall, remove spent flowers to prevent seed set. Keep the plants fertilized until the ground freezes to promote root system establishment. After the soil freezes, mulch the plants to prevent alternate freezing and thawing cycles.