June or July to Visit 美东(纽约, Boston, 大瀑布, Maine) + Canada (Toron

We (Parents + 3 little kids) plan to spend 3 weeks this summer visit East Coast (纽约, Boston, 大瀑布, Maine...) and Canada (toronto, Montreal...) by driving our car.

Any suggestion when should we do it, June or July? Is it true that it will be more crowded in July compared with June? Any drawback if we do it in June?

Thanks and Happy Summer!