In spring. But just between us, I have never split mine and they are going strong 17 years later. I've never split my daffodil bulbs either and they are gorgeous.

You can divide them now, if you need to. I divided day lilies and hostas this weekend, and they'll probably do fine.

I've got so many daylillies, they're like weeds. Anymore, I just divide them when I've got time. They seem near impossible to kill. They only bad that that's happened to mine is they might not bloom the same season I divide them. But, the ones I've divided this early in the spring have always bloomed the same season.

You can split them soon after blooming.
I dig up the clumps and just cut them into pieces with a garden knife. A minimum of 2 fans per clump will help them re-establish faster. Just water and mulch well.

Now that the foliage is finally out, we see our daylilies have officially gone bonkers and need to be split up. i think the time for this is in the fall. does anyone have particular recommendations from experience ? we are researching on the web and books, but thought I would reach out for personal advice as well.

thanks!