America celebrated its National Day of Jubilee, or 100th birthday, on July 4, 1876, then-President Ulysses S. Grant declaring it a “day of reflection and gratitude” as patriotic celebrations broke out across the rapidly growing country that was still reeling from the Civil War.

“The Centennial Anniversary of the day on which the people of the United States declared their right to a separate and equal station among the Powers of the Earth seems to demand an exceptional observance,” Grant said in his June 26, 1876, proclamation encouraging religious services to mark the occasion.

“It seems fitting that on the occurrence of the hundredth anniversary of our existence as a nation a grateful acknowledgment should be made to Almighty God for the protection and the bounties which He has vouchsafed to our beloved country,” the Union army hero wrote.

As the country barrels toward its 250th year, many things have changed, but Ulysses Dietz, the youngest great-great-grandchild of Ulysses and former first lady Julia Grant, told The Post his historical ancestor would have a lot to be proud of — and find many things surprising.

“I think a lot of great things have been achieved that he would be pleased at because they were part of his vision for what could be,” Dietz said of his great-great grandfather, who championed civil rights during Reconstruction.

“I think he would also see a lot of bad things that were also bad when he was president and be disappointed that we haven’t overcome that,” he added.

Surprisingly, Dietz, 71, the now-retired longtime decorative arts curator of the Newark Museum of Art in New Jersey, said his famous lineage wasn’t a major part of his upbringing, and that his grandfather, Ulysses, from whom he inherited his robust moniker, never really sat him down to talk about it.

“I’ve always been sort of sorry about that, he died when I was 12. But the family never really talked about it,” he lamented, pointing out that their ambivalence at the time could be explained by Grant’s reputation being at a bit of a low point in the 1960s, “mostly through misinformation.”

Despite seeing his great-great grandfather’s face every time he looked at a $50 bill, it wasn’t until decades later that the full weight of his family’s connection with history really hit him.

The Maplewood resident was invited to give a speech at the General Grant National Memorial — better known as Grant’s Tomb — in Morningside Heights in 1987, and pretty much winged it.

“I went in with no preparation or knowledge, I don’t even remember what I talked about. That was sort of a wake-up moment when I thought, you know, if they’re gonna ask you to do these things, you better start learning.”

Dietz is the only one of his family’s generation — 41 people — named Ulysses, all the more reason to learn more about his namesake.

Born Grant Ulysses Dietz, he changed his name to Ulysses G. Dietz at the age of 15, while attending Exeter boarding school (as Ulysses Grant Jr. did).

By the time he got to college, everyone knew him as Ulysses.

“It was the ’70s and having a weird name was cool,” Dietz explained. “It wasn’t central to my being, just part of my identity.”

But after the 1987 event, he started voraciously reading everything he could get his hands on to learn his family backstory, and has returned to make a speech at Grant’s Tomb every April 27 — Grant’s birthday — for the last 40 years.

As for what Grant would be most proud of 150 years after he marked the celebration of America’s 100th birthday, “I think he would see a country that was pluralistic and ethnically diverse and religiously diverse, where people have opportunity and everybody can go to college, anybody can be president,” Dietz said.

“I think he would be very proud of that.”

He said Grant, who supported the 15th Amendment giving black men the right to vote and prosecuted the Ku Klux Klan while in office, would be “thrilled” and “surprised” by the strides the nation has made around pluralism and diversity.

Looking back on 250 years of American history as a whole, Dietz says he’s struck by the vision of the Founding Fathers in laying the groundwork that got the country to where it is today, starting with the Declaration of Independence.

“It’s sort of remarkable looking back that this group of elite, white landowners, many of whom were slaveowners, put together this document that withstood the test of time and then followed it up with the Constitution that has also withstood the test of time,” he said.

“I think it’s remarkable that these men, without understanding where the country would go, came up with ideas that were every bit as important and relevant now as they were 250 years ago,” he added.

“Honoring that commitment to freedom, life, liberty and the pursuit of happiness — and still improving on it — is something we should be very proud of.”