In our climate I prefer to dig,divide and replant in the fall, after the heat of summer, and before the onset of winter. However daffodils are very tolerant and will survive if they are done anytime they are dormant. Al

I do mine when the foliage is brown (and I can still find them).

Like laceyvail, I do it when the foliage is almost gone. By fall, I've forgotten where the danged things are, if I remember that I was intending to divide them at all.