The financial markets should start preparing themselves for an October surprise.

October, of course, has always been a tricky month for Wall Street. But the markets would be wise to prepare ASAP for a surprise — because it could come in September or even this month, depending on when Russian President Vladimir Putin decides to make trouble.

But there’s so much more to this story and “the surprise” because there are, in fact, a number of surprises that are possible.

You’ve already heard stories — including one published recently by Politico — about how the Democrats are worried that Putin will try to interfere with the US presidential election by releasing the missing personal emails of Hillary Clinton.

But Putin and his gang of spies aren’t the only ones with those emails, according to a very reliable source of mine in the intelligence community.

The NSA — the National Security Agency — also has copies of Hillary’s private emails.

How did the NSA get them?

Earlier this year, the Russians hacked computers at the Democratic National Committee and the Republican National Committee. Neither party knew it had been hacked until the NSA alerted it.

Some emails from the DNC’s computer came out right before the Democratic convention in July and they created a stir, because they showed that forces aligned with Hillary did their best to deny the party’s nomination of Bernie Sanders.

So far, no Republican Party emails have come out — and they aren’t likely to.

One reason may be that Putin — as others have said — seems to favor Donald Trump over Clinton. Another reason might be that the Republicans’ emails will likely show the party conspiring against Trump — which doesn’t matter much now that he’s the nominee.

The Russians got Hillary’s email more easily. As I’ve written before, they stole the password for Madame Secretary’s BlackBerry probably during one of her 2012 trips to that country.

Once they had the password, the Russians could see everything that Hillary was emailing and could record every conversation on that non-secured line.

And the NSA, all along, was watching the Russians monitor Hillary. That’s how the Adm. Michael Rogers-led NSA, which specializes in global monitoring and collection of data, came to have Hillary’s emails, according to my source.

What is in Hillary’s emails is anyone’s guess — except, of course, for the Russians and the NSA employees who have seen them.

The NSA even offered the emails to the FBI when it was conducting the probe that ultimately found that Hillary was “extremely careless” with sensitive government information. The FBI declined the NSA’s offer, I’m told. The emails, the FBI contended, would have been useless in court if Hillary had been indicted because of the way they were obtained.

The NSA would never officially release the emails, but that’s not to say a rogue agent might not leak ’em. There is, I’m also told, bad blood between Hillary and the NSA that goes back to her early days as secretary of state.

One of the NSA’s responsibilities is to make sure government agencies like the State Department have safe areas where sensitive and classified information can be discussed and stored. These are called Sensitive Compartmented Information Facilities, or SCIFs.

When a meeting is held inside one of these SCIFs, spies can’t snoop. And BlackBerrys don’t work inside a SCIF.

Hillary, my source says, requested that a non-secure room be built near the State Department’s Washington SCIF so that she could use her BlackBerry.

The request was approved — but over NSA protests. And that started a feud between Hillary and the NSA that only got worse with the Benghazi attack that killed Ambassador Christopher Stevens and a “foreign service information manager” — likely an NSA employee.

What is in Hillary’s emails is anyone’s guess — except, of course, for the Russians and the NSA employees who have seen them. But they probably include some very personal, embarrassing stuff as well as talks about Clinton Foundation donations.

Clinton has said her missing 30,000 emails were about yoga and a wedding. Maybe there’s some of the email chatter that we all do with friends. But if that’s all, why were the emails erased in the first place?

If someone at the NSA decides to leak some of Hillary’s emails, it might not just be for revenge. There could also be a very reasonable concern that if she wins on Nov. 8, Hillary could be blackmailed by the Russians if those incriminating emails are still private.

The NSA could justify its actions — at least to itself — as having to get in front of a potential problem.

Call it “preventive leaking.”

There is also a surprise that the Democrats could pull on Trump.

For example, it wouldn’t be too shocking if the IRS suddenly finished up its inquiry of Trump’s taxes and put him on the spot.

Trump has refused to release his tax returns. And if they show, as some people think, that he’s not as rich as he claims or has evaded taxes, Trump will have issues to deal with.

But that wouldn’t really be a surprise for Wall Street in a negative sense since — if there’s no counter-surprise against Clinton — that would merely confirm that Hillary is going to be our nation’s first woman president. And Wall Street seems to have accepted that already.

October has been full of market jolts — the 1929 crash, Black Monday in 1987, the mini-crash in 1989, etc. — and some Hillary news could rock stocks again.

That’s why they call them surprises.