Memorial Day, not to be confused with Veterans' Day, honours those people who have died while serving in the military, whereas Veterans' Day, which is observed on November 11, has come to honour everybody who has ever been a member of the Armed Forces, whether they died or not.  Memorial Day has been observed on the last Monday in May since it became an official holiday in 1971. Before that it was celebrated on May 30... well... if you happened to live in the North. (Memorial Day 2020) 

The idea of routinely maintaining the gravesites of your ancestors – a Decoration Day – is quite old.  Before the Civil War, families would generally clean up gravesites toward the end of the summer, using the occasion as an excuse for family reunions.  After the Civil War, there were suddenly a lot more fallen soldiers to honour.  As a result, many secular, patriotic ceremonies sprang up all over the country.  (Memorial Day 2020)  Where they sprang up first, though, is a bit contentious.

Waterloo, New York, is credited with holding "the first formal, village wide, annual observance of a day dedicated to honoring the war dead," which they called Decoration Day, on May 5, 1866.  Congress made the declaration official in 1966 when they recognized Waterloo as the birthplace of Memorial Day.  If you're ever in Waterloo, there's a museum. (The History and Origin)  The observance was moved to May 30 in 1868, in particular because it didn't mark any battle. By 1890, all the Northern states had made Decoration Day a state holiday. (Memorial Day)

Saying that Waterloo, New York, is the birthplace of Memorial Day, however, might be a bit of Northern revisionist history.  The first Confederate Memorial Day, which was simply called Memorial Day, was observed on April 26, 1866, in Columbus, Georgia, one month before it was celebrated in Waterloo, and a full two years before it became a truly official holiday in the North.  April 26 marks the anniversary of when most Southerners considered the Civil War to have ended, when General Johnson surrendered to General Sherman at Bennett Place, North Carolina. (Confederate Memorial Day)  Indeed, there are those who claim that the observance of Memorial Day in the North was a direct response to its taking place in the South.  The day was not referred to as "Confederate Memorial Day" until after observances became established in the North. (Confederate Memorial Day)

However, even claiming that the Southern states started Memorial Day might be a bit of Southern revisionist history.  The first recorded observation of Memorial Day was in May of 1865 – a year before the Southern observance – by freed slaves in South Carolina. (Memorial Day)

Confederate Memorial Day is still celebrated throughout the South on various days in the spring, depending on what state, and even which part of that state, you might happen to be in.  It is still an official holiday in South Carolina, Mississippi, Florida, Alabama, and Tennessee.  (Confederate Memorial Day)

After the First World War Memorial Day became less of a "North / South" thing, when it was expanded to include all soldiers who had fallen in any American war. (Memorial Day 2020)

Since the end of World War I, it has become a tradition to wear a single red poppy to honour the dead on Memorial Day.  Poppy seeds are scattered by the wind, and they tend to lie dormant in the ground, only germinating when the ground is disturbed, as it was in a big way during World War One.  Poppies, therefore, are usually one of the first things to appear on a battlefield, even before the fighting has stopped. (Memorial Day 2020)

John McCrae is generally credited with starting the poppy tradition.  McCrae, who witnessed the First World War, wrote the poem "In Flanders Fields" in 1915, which features the line "In Flanders fields the poppies blow / Between the crosses, row on row...." (McCrae)

Inspired by McCrae, Moina Michael wrote her own poem, "We Shall Keep the Faith," in 1918 about Flanders fields and the poppies that grew there. The poem features the line, "And now the Torch and the Poppy red / We wear in honor of our dead."  (Michael) 

Wearing of poppies to honour the war dead quickly spread throughout the known world, especially in Europe.  It also spread to Veterans' Day, where it has come to symbolized not only the dead, but the hope of recovery and new life. (Memorial Day 2020)

On Memorial Day, people traditionally place flags on the graves of veterans.  As well, there is a National Observance at 3:00 p.m. local time.  And then there are the barbecues and picknicks.  Memorial Day, aside from honouring the dead, has become the unofficial official start of summer.  (Memorial Day 2020)