I have bought a number of various 'garden ready plugs'. When will it be safe for me to plant them out? Is there a safe date or is it all a game of chance? I live in the Boston area.
I think 'garden ready ' is a very misleading term. It depends what the plants are and what your own conditions are.
End of May/early June is when it's safer to plant out anything which isn't hardy, but if you buy plugs, I'd pot them up for a while till they're bigger if you plan on putting them in the ground. If they're going in pots - as a display - you'd get away with it if they're well grown and healthy.