Other Twitter users accused Trump of "making everything about himself" and having a "boundless" ego.
The mother of a 25-year-old Marine who died in Afghanistan railed against the use of the phrase "Happy Memorial Day" just hours before the President's tweet. Denise Torbert told her local newspaper in Lancaster Country, Pennsylvania: "It makes it harder. It's not happy," she said. "Say 'Have a nice day' or whatever. But not 'Happy Memorial Day'.
"There's nothing happy about it."
Others lambasted the President as a Vietnam War "draft dodger". Trump's record was revealed in 2011 after a Freedom of Information Act request by the website the smokinggun.com. He twice deferred while studying at Fordham University in the Bronx, and twice more when studying business at the University of Pennsylvania.
After graduating in 1968, his status became "available for service" and a physical examination that July showed he was in suitable health. But he had another exam three months later, which gave him a new, low classification that meant he would only be called up in a national emergency. Later, Trump said his issues were related to bone spurs in his heels.
All this from a leader who in 2015 criticised John McCain's service record with the comment: "He's a war hero because he was captured. I like people who weren't captured."
Memorial Day takes place each year on the last Monday in May, and began to commemorate fallen soldiers after the American Civil War. The US observes another public holiday on November 11 to honour people who served in the nation's armed forces — Veterans Day.
Half a century ago, a veterans group commander General John Logan named the day "Decoration Day", and it became a tradition to lay flowers and wreaths on soldiers' graves.
The President was to visit Arlington National Cemetery to lay a wreath at the Tomb of the Unknown Soldier.
The monument, across the Potomac River from Washington DC, symbolises all soldiers killed in war. Similar tombs are found in many nations including the National War Memorial in Wellington. The concept was invented in Britain and France after World War I.
Reverend David Railton had seen a grave marked by a cross reading "An Unknown British Soldier" while serving in the British army, and suggested a symbolic funeral and burial for an "Unknown Warrior" to remember all soldiers whose remains were never identified.
Melania Trump posted a less controversial tweet than her husband thanking "all the service members & their families who sacrifice so much to keep us safe".
She noted that "freedom isn't free", a phrase widely used in the US to thank servicemen and women for their sacrifice.
