If your daylily is a large clump and the flowers are getting smaller it's time to divide.Alternatively you can wait until the Autumn at least 6 weeks before the frosts start so that the replant can get established.
Perhaps of no real interest, but the Daylily may be reclassified as a noxious weed -(once you"ve got it, you've got it!!) Although I'm not sure if this includes the thousands of hybrids.
Here, in the western U.S., ours are still dormant, although the ground has thawed. This is the best time to divide them since they have all season to grow and store energy for winter... I would say, however, there's an old rule of thumb to divide any plant opposite it's blooming season.
I've had good luck though, at early spring dividing...