I planted mine as soon as I could count the first set of true leaves. They've lived through snow and sleet and record cold temperatures in the teens.

I used the HOS method too. Here are some dianthus.

{{gwi:360251}}

They've grown a little since then.

Assuming you are talking lettuce and spinach, and the soil in your cold frame is thawed plant them now, but I would also plan on harvesting them from there too because they won't want to be transplanted again once they've become established. I'm doing the same thing but the soil in my covered raised bed is still frozen and the seedlings don't have their true leaves...but as soon as they do I will be transplanting. Make sure you leave enough space between seedlings...like 3" minimum if you will be harvesting baby greens.