According to Newsweek, there was actually a Memorial Day post before that one [bolding mine]:
In a post on Truth Social, Trump, the presumptive 2024 Republican nominee for president, shared an image showing himself saluting a grave and said, “We can never replace them. We can never repay them, but we can always remember. Today, that is what we are doing - we remember.”
But as for your quote, Newsweek says there was more after that:
“Happy Memorial Day to All, including the Human Scum that is working so hard to destroy our Once Great Country, & to the Radical Left, Trump Hating Federal Judge in New York that presided over, get this, TWO separate trials, that awarded a woman, who I never met before (a quick handshake at a celebrity event, 25 years ago, doesn’t count!), 91 MILLION DOLLARS for ‘DEFAMATION,’” Trump wrote in the post.
"She didn’t know when the so-called event took place - sometime in the 1990’s - never filed a police report, didn’t have to produce the ‘dress’ that she threatened me with (it showed negative!), & sung my praises in the first half of her CNN Interview with Alison Cooper, but changed her tune in the second half - Gee, I wonder why (UNDER APPEAL!)?
“The Rape charge was dropped by a jury! Or Arthur Engoron, the N.Y. State Wacko Judge who fined me almost 500 Million Dollars (UNDER APPEAL) for DOING NOTHING WRONG, used a Statute that has never been used before, gave me NO JURY, Mar-a-Lago at $18,000,000 - Now for Merchan!”
For contrast, here is Biden’s message:
Since America’s founding, our service members have laid down their lives for an idea unlike any other: the idea of the United States. Today, as generations of heroes lie in eternal peace, we live by the light of liberty they kept burning. May God bless them, always.