Maria, Best to wait until all frost danger is past, but if you want to give dahlias and such a headstart you can pot them up for a few weeks now in potting soil and leftover nursery pots, and grow them indoors for a few weeks. I just put my cannas, for instance, which I had overwintered in the basement, into nursery pots and tucked them in the garage (which isn’t heated but stays cozy compared to outside at night). I drag them out into the sun in the daytime, then back in. If you just had a few pots, you could do it indoors in the house in a warm spot. If you want to just wait and plant out directly, give it another couple of weeks–at least Memorial Day "in the Berkshires" (as per your online name) or even first week of June. M.