Each mum you choose should be labeled as to when it blooms in the fall. The tag will say either early, mid or late. Very few late mums are grown in our area because our season is too short. Sometimes it will be indicated by E, M or L. Mum season extends at least 6 weeks, from mid-August to late September. Since mums that have been grown outdoors can take frost, you will often see mums blooming long after your other annuals and perennials have died back.
Mums are naturally fall bloomers.
Their blooming is related to the nights getting longer. Buying them in bloom in spring enables you to choose by forms and color early in the season. If you let them, these plants will bloom for a few weeks early in the season, often exhausting the plant. Ideally, you want to pinch off all the blooms and buds when you buy the mums to encourage vegetative growth and root development for fall bloom. By pinching a week or two later than normal, you can make an early mum bloom with the mid-season varieties.