BY JULIA GLUM Memorial Day may be the unofficial start of summer, but it's so much more than an excuse for a three-day weekend. It's a U.S. holiday with a lot of history and lasting significance. Held annually on the last Monday of May, Memorial Day dates back to the Civil War. Who exactly created it and where are unclear, but legend has it that the tradition began in 1864. That's when three women from Boalsburg, Pennsylvania, put flowers on the graves of loved ones who had died in the war, including a young man killed in the Battle of Gettysburg. By 1865, according to the Library of Congress, people in states such as South Carolina and Mississippi were participating in unofficial ceremonies that preceded Memorial Day. But Waterloo, New York, claims to be the birthplace of the holiday, given that it held what Nelson Rockefeller would later call the "first, formal, complete, well-planned, village-wide observance of a day entirely dedicated to honoring the wa