with data from the past 40 years shows ave low end of April is approx. 40 in Portland and 47 in Boston.
End of May shows average lows of 51 in Portland and 57 in Boston. Knowing the range might be informative.
Based on my 40 years in western NY I would not start before May 15 unless you are can handle miserable nights setting and tearing down camp in cold and possibly wet conditions. Sleeping in a warm bag is the easy part...setting up camp, cooking, and packing in the AM I find is the test of handeling the misery.