what plants do you have in mind? It depends on what plants need dividing as to what time they should be.

As kildermorie says, it totally depends on what plants. You could ask about individual plants. As there are a number of perennials that hate to be disturbed and even digging up to split can be lethal to them. And then of course there is the flowering time that also affects things. Some perennials flower early so could be split when they have finished flowering and going over. or there are the plants that flower now, that should be split is spring.