It’s that time again, friends. Mid-September. And I don't mean it's time to set your fantasy-football lineup.

I mean it’s time for the first edition of our annual September History Watch series, highlighting lots of fun little historic pursuits you may not realize are in progress. And there’s only one place to start this September.

In Flushing, NY. Where else?

I don’t know about you, but I can’t decide which is more ridiculous -- Yoenis Cespedes or that team that now employs him, those New York Mets. You decide.

The greatest U-turn in modern history

A mere eight days before the Mets traded for Cespedes -- back on July 23, 54 days ago -- they started a lineup that seems downright incomprehensible now.

Their cleanup hitter was batting .170, with a .565 OPS. That was John Mayberry Jr. Their No. 5 hitter was batting .179, with a .587 OPS. That was Eric Campbell.

The Elias Sports Bureau told us that day that it was only the second time in the live-ball era that an actual major league team started No. 4 and No. 5 hitters who both had batting averages that low, after least 100 at-bats. And the only other team to do it -- the 1975 Oakland Athletics -- did it the day after they clinched first place! So they were barely trying to field a lineup.

So “offensive embarrassment” would be a polite way to describe the Mets before they traded for Cespedes moments before the July 31 trading deadline.

But since this is a history lesson, the hard facts are these: From April through July, the Mets were the worst offensive team in their league. Last in runs scored. Hitting .234 as a team, with a .662 OPS. Averaging 3.5 runs and 0.9 homers per game.

And then they did the greatest legal U-turn of modern times.

Since they flipped the calendar to August, the Mets have somehow turned into the best offensive team in their league. First in runs scored. Hitting .275, with an .840 team OPS. Averaging 6.2 runs and 1.7 homers per game. Holy schmoly.

So how often has anything like that happened, you ask? Pretty much never. Thanks for your inquiry.

According to Elias, if you don’t count strike years, only one other team since 1900 has been last in its league in runs scored from Opening Day through July and then first in runs scored from Aug. 1 on. And that team isn’t exactly a perfect comparison.

It was a 1985 Cleveland Indians juggernaut that traded for nobody at the deadline, had no late-season call-ups of note and wound up losing 102 games.

But that’s it. So if the Mets keep this up, they’ll be the only National League team in history to score the fewest runs in the league through the end of July and the most from August on. Whaddaya know.

In a related development, it’s time to turn our attention to ...

Cespedes vs. the Crime Dog

Back in his previous life, Cespedes hit 18 home runs for the Detroit Tigers this season. Then he showed up in New York and turned into Reggie Jackson.

If you’re calculating along at home, you know that Cespedes only needs one more homer as a Met to equal his total as a Tiger. And that’s not just ridiculous, it’s potentially historic.

According to the Sultan of Swat Stats, esteemed home run historian David Vincent, just five other players in history have ever hit 17 home runs or more for two different teams in the same season. And here they are:

But you’ll notice something about that list: Cespedes has a chance to do something only one other hitter in history has ever done: hit at least 18 homers for one team, then get traded and hit more homers for another team in the same season.

Even the Crime Dog himself, Fred McGriff, ever did that -- thanks to his fabled Out of the Fire Sale Into the Fire trade from the Padres to the Braves on July 18, 1993.

But what would set Cespedes apart from McGriff or anyone else is the late date of his deal to the Mets. McGriff got only 47 fewer at-bats as a Brave than he got in San Diego back in ’93. But even if Cespedes plays every day for the Mets the rest of the way, he won’t come within 150 at-bats of the 403 he got in Detroit. And he still has a chance to hit more bombs for team No. 2 than team No. 1. Wow.

So are you catching our drift here? If you’ve been thinking, “Boy, I’ve never seen anything like Yoenis Cespedes” or “Wow, I’ve never seen a team make a turnaround like the Mets,” well, guess what? You ain’t hallucinating.

Because this, friends, is the stuff the September History Watch is made of.