For most of them, about 2 to 3 weeks before your expected last frost. Here in my part of Michigan, that translates to around the 10th to 17th of May.

By the time they emerge, no more danger of frost (as a rule).

You can warm the soil and help them along by planting them and then throwing a piece of black plastic over the planting bed (poked full of holes for water to penetrate) for about a week -- really warms the soil.

I already planted my cannas. I am waiting until the ground has warmed up before planting my Dahlia. I usually plant them well after Mother's Day.
You can plant Iris now. It's a perennial anyway.