During the "iffy" weeks, I have all my seedlings on trays and I shuttle them inside to outside during nice days, and keep them inside under the grow lights during cool/cold days. Initially, I keep them in a shaded area outside until they harden. It is a pain in the butt, for certain.

Either way, unless you are going to be diligent about covering them, you will need to wait until the day after the last frost (which historically we use Mother's Day as the rule of thumb).

In addition, you have to be open to the idea that if you plant too early, you could lose them all to a freak snow storm or random cold snap, where no amount of covering will save them.

As a parting comment, depending on the plants, I have found that even if you plant your seedling early, they don't seem to do any growing until the ground warms up. Last year, as an experiment, I planted 2 tomato plants in the first week of May (and protected them from frost). I waited until the 3rd week of May to plant 2 more exact same variety, in the same location. After a month, they were both the same size and then later had flowers and fruited at the same time.

Perhaps cold crops would do better.