Since I haven't been moved to write very much of late, I have promoted last year's Memorial Day article to first post for this year instead.
Memorial Day commemorates those US men and women who have DIED while in military service. It is not a day to honor those who served and lived (Veteran's Day), or a day to remember those killed in attacks on civilians (9/11). It has been suggested by David Blight, professor at Yale that Memorial Day has its origins at a ceremony performed by former slaves in May, 1865 to honor UNION dead. During the day long ceremony, bodies were exhumed from mass graves at the old Washington Race Course (the racecourse had been turned into a prison camp during the Civil War) in Charleston, SC and re-interred properly. There may be no connection between this touching ceremony and the observance in Waterloo, NY the following year which is often credited as the first Decoration Day (so called because the graves of Union soldiers were decorated).
This first official observance was made by General John A. Logan which proclaimed in General Order No. 11 on May 5th, 1868 that the 30th of May would be observed as Decoration Day.
The 30th day of May, 1868, is designated for the purpose of strewing with flowers or otherwise decorating the graves of comrades who died in defense of their country during the late rebellion, and whose bodies now lie in almost every city, village, and hamlet churchyard in the land. In this observance no form or ceremony is prescribed, but posts and comrades will in their own way arrange such fitting services and testimonials of respect as circumstances may permit.
You may note that these three origins for Memorial Day all point to the decoration of the graves of Union soldiers. Not surprisingly, most of the southern states refused to celebrate it. Mississippi, which is the final resting place of a considerable number of union dead did observe it. Hardly surprising then, is the fact that some southern states celebrate a different date - known as Confederate Memorial Day (of course now days this holiday is celebrated in addition to Memorial Day proper). Celebrated on either the 4th Monday in April or on May 10th the day gives an official nod to the Confederate dead of the US Civil War in much the same fashion as the (subtly) Union-centric Memorial Day.
Over the years Americans have forgotten some of this old enmity and most ignore (or are ignorant of) the distinction between Union and Rebel forces on Memorial Day. The day became more commonly celebrated after World War II. The name "Memorial Day" was first used in 1888 but only made official in 1967 which also ensured the day's longevity by moving it by law to create a three day weekend. There have been several attempts to move it back to its original date (the 30th) because the three day weekend "cheapens" the holiday, but such movements (understandably) haven't met with very much success.