Sandi, are they wintersown or grown under lights? For my wintersown stuff I almost never pot up to a bigger pot- I stick everything right in the ground. Last frost date doesn't matter if they're already used to the cold. At most you might "harden off" to the wind by removing the top part of the jug for a couple of days, then stick into a garden bed as long as your soil isn't frozen and you can dig.
As you can see from my lupine photo, looks like second true leaf was just starting when I transplanted. If yours are indoor seedlings you might want to pot up and harden off when possible. If they're wintersown, they don't need to be hardened off as they were born into the cold and they're used to it so they could go right in the ground.
Honestly, wintersowing is just so much easier in so many ways I don't understand why more people don't do it.
Karen
