(image from here )
Wikipedia states that Memorial Day, "formerly known as Decoration Day, it originated after the American Civil War to commemorate the fallen Union soldiers of the Civil War." The official holiday name change wasn't until 1967 (the bill also moved the date of the holiday from May 30th to the last Monday in May). The flag is flown at half mast (after being raised to the top and then lowered) until noon, when it is then raised all the way.
(please note I am paraphrasing information found on the website, unless it is in quotations)
So, as you head out to the pool or a barbeque, please take a moment to remember those who fought and died for the freedoms we enjoy.