when to devide Hellipores?

Once established, hellebores are not really keen on root disturbance. If you have to do it, it should be done after flowering finishes. Give a heavy feed after division

Andrewr is quite right, they do not like being divided. If you want to increase them don't weed around the plant and you will soon find plenty of seedlings, pot them up and wait for 2 years or so and you will be surprised, you could even have some different colours.

I am going to have a moan, here! My Hellebores don't seed themselves, apart from H. foetidus!

I agree with Andrew and Wyeboy - you'd be taking the risk of losing them if you do divide them.