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You could try Wikipedia for aYou could try Wikipedia for a start but there are numerous others. Or you could consult any number of books written over many centuries on the subject. Where Mayday is celebrated in the Northern hemisphere it is celebrated as a Spring festival and May is a Spring, not a Summer month. It is as simple as that. "Mid-Summer" does not imply the exact calendrical day between the beginning and end of Summer, any more than "mid-Atlantic" implies the exact mid-point between Europe and America. Most humans do not think in such precise ways. They are not computers.

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?