The right time to divide day lilies is in September, after the plant finishes blooming for the year. This allows the plant to establish roots before winter.
While it is best to divide day lilies at the end of summer or beginning of fall, it is also fine to divide them at any time during the spring and summer growing season. It is time to divide a day lily when the plant produces fewer flowers and stalks than normal and the plant clump looks sparse in the center. An overgrown day lily plant looks weak and too large for its garden bed.