Well, I would determine when your average date of last frost is and then plant 1-2 weeks after that. Just so the leaves do not get zapped by late frosts. Unless you can somehow protect them if a late frost rears its ugly head.

What kind of hydrangea? There are 4 species commonly found in gardens in your zone, and IME they have different tolerance to frost.

Is the soil dry enough to be workable? Digging in really sodden soil can effect the soil structure.