What a super sesquicentennial is shaping up for Saturday. That’s the 150th running of the Belmont Stakes, where all eyes will be on Justify, the contender for the Triple Crown.

Just don’t fall in love, I say.

It’s true that Justify slopped through the mud to win both of the Triple Crown’s first two legs, the Kentucky Derby and the Preakness. And, true, rain is predicted for Saturday.

Beware, though, of what happened in the astonishing last four seconds of the Preakness — when, from nowhere, a horse named Bravazo lunged out of the fog. He was half a length behind Justify and accelerating on him when they hurtled over the finish line. He’d have nosed him with another few yards.

Which is just what — an extra 3/16ths of a mile — the Belmont is about. It’s the longest and hardest test of the Triple Crown.

It’s a heartbreaker.

What a glory for New York, though, where racing has a long history. Saratoga, one of its jewels, opened in 1863, Aqueduct in 1894 and Belmont in 1905.

Today, New York accounts for roughly a fifth of all money bet at thoroughbred tracks in the country. For the home of the Triple Crown, an elegant expansion is planned.

After the Belmont settled at a mile and a half, it came to be known as the Test of Champions. Seventeen horses have won the first two legs of the Crown only to fade at Belmont.

That happened dramatically in 2008, when a magnificent giant named Big Brown arrived at Belmont. He had convincing victories at the Derby and the Preakness.

I brought two of our children out to Belmont and stood in the sweltering mugginess. Big Brown went off the favorite, at odds of 3-10. We were going to watch history.

Yet once on the track, Big Brown failed to respond. The crowd was dumbstruck. Before the top of the stretch, the steed was pulled up, possibly for a loose shoe.

Big Brown turned out to be the only Triple Crown contender to fail to finish the Belmont outright. And a lesson for the children, not to mention me.

The greatest Belmont ever run was undoubtedly in 1973, when Secretariat arrived having broken the race records in both the Derby and the Preakness. The big red steed ran the first half of the Belmont neck and neck with a horse called Sham. They were way in front of the other three horses in the race.

Then something incredible, almost eerie happened. The camera panning across the infield lost them momentarily as they disappeared behind an American flag.

I have half-joked that at that moment, God reached down to touch Secretariat. He suddenly started pulling away, moving like, the announcer exclaimed, a “tremendous machine.”

By the time Secretariat barreled into the top of the stretch, he was 18 lengths in front. He would almost double that distance by the time he careered over the finish line.

Stupefied, the crowd had fallen almost silent, gasping in amazement. Many, it was widely reported, began weeping. Secretariat’s record has never been approached.

Nor is it likely to be on Saturday. The news this week trumpets that the post for the Belmont landed Justify on what one headline writer called the “dreaded rail.”

That means he’s going to start closest to the inside fence, which his trainer, Bob Barrett, doesn’t like. It’s not dispositive though.

Once a whacky horse ran to the outside rail — and still won. Eventually, in 1941, that horse, Whirlaway (“Mr. Longtail”), went on to win the Triple Crown.

Which brings me back to the danger of Bravazo. In the last moments of the Preakness, shrouded in fog, he seemed to come out of nowhere.

In just two seconds, he rocketed past Lone Sailor, Tenfold and Good Magic like they’d stopped for groceries. He finished second by less than half a length.

With the added distance of the Belmont, he’d have overtaken Justify. That’s not a prediction — just a warning, though even that is with a caution.

I issued a similar warning in 2014, when California Chrome was nearly overtaken by Commanding Curve late in the Derby and by Ride on Curlin in the Preakness.

At the Belmont, none of them was in the top three. Which is what, no doubt, makes horse racing. Here’s to another 150 years at the Belmont.

This column first appeared in the New York Post.