While the weather will be favorable May thru late Sept, and the scenery full of life, Maine has lots of black flies thru May and June, which go away in July-ish. But then there are mosquitos . Neither of these biting insects like being out in the sun, but are pretty prevalent in woodsy shaded areas or at dusk. Still, mosquitos tend to be more bearable than the flies. Along the coast the mosquitos aren't that bad, also.

It is very cool most nights, even in the high summer. I don't know if you'd camp, but if that is of interest, going more toward July or August might be more pleasant. In late July and August you can probably catch lots of blueberry and raspberries along the roads too.

And of course, fall is well, amazing in New England or NS, and usually quite dry. Much cooler temps and possibly fewer people.

HTH, a little. For a summer trip, I guess I'd pick late July. For a fall trip, I'd pick mid Sept - or basically when Denise is going!