When to plant your perennials depends on where you live and the planting zone as well as the perennials you want to plant. Contact your state extension service for some suggestions. You will find that information online.
I would do now...getting ready to start myself in Tennessee!
After the danger of frost is past.
What zone are you in? Do you know your last frost date?
depends where you live?? Old Gardening rule "do not plant til after Mothers Day" even later/after, some parts of country as you know they are getting snow again/still. What are you planting ?