Emily, I am around the same time as you and I use "Memorial Day" as a base. I also set out plants on a hardy bases. Such as plants that are tough and can take a bit of cold go out first the more tender plants like impatient go out last.
I have a yard that has lots of trees so if they are fully leafed out they protect the plants from frost kill. My friend whose gardens are out in the open with no trees has to plant out later then me.
I also have old bed sheets stored in the basement just in case.
It amazes me how many people get in a rush to set out plants just because the box stores are selling them and then return later to buy replacements after the frost gets to them.
