It’s not very often that you’ve got two different Black Fridays occurring on the same day. But that’s what happened this past Friday.

And despite the stock market’s substantial rise Monday, there’s an important message — and an important warning — that the less obvious Black Friday, which I’ll call BF II, delivered to those of us who have a significant portion of our net worth invested in stocks.

For almost 70 years now, Wikipedia says, the term Black Friday has been used to describe the start of the Christmas shopping season on the day after Thanksgiving.

BF II takes its name from the fact that the U.S. stock market got clobbered Nov. 26, with the Dow falling more than 900 points (about 2.5 percent) and the S&P 500 and Nasdaq markets each falling by about 2.2 percent. In terms of money lost, my favorite metric, the FT Wilshire 5000 Total Market Index, fell by about $1.2 trillion, which was its biggest dollar loss since Oct. 28, 2020, when it fell $1.25 trillion.

Painful memories of BF II may already be fading for many people, given Monday’s 236-point rise in the Dow; the S&P and Nasdaq recovering 57 and 82 percent, respectively, of their BF II losses; and the Wilshire rising by $525 billion.

These increases happened even though the new omicron variant of the coronavirus looked a lot more dangerous to the United States on Monday than it did on Friday because we now know that it has shown up in North America.

On Friday, omicron was cited as the cause of the sharp decline. On Monday, omicron looked worse — but the market looked a lot better than it did Friday. As often happens, memories on Wall Street have proved to be short. At least for now.

But forgetting BF II is a serious mistake, because there are lessons there that investors ignore at their peril.

Since the market’s bottom about 20 months ago — on March 23, 2020 — the Wilshire has more than doubled, and as of Monday’s close was up 113 percent, increasing U.S. stocks’ market value by $26.7 trillion.

Week after week, we’ve seen stocks make one high after another, and people who’ve put every dollar they could lay their hands on into stocks have done extremely well.

But the market’s drop last Friday shows why it’s wise to keep a reasonable amount of cash on hand — “reasonable” being a flexible term — rather than putting every penny you own and can borrow into stocks.

That’s because if you’re going to own stocks for the long term, you need mental and financial staying power. Imagine if Monday had been a Black Monday, followed by Black Tuesday, Wednesday, Thursday and Friday, with declines similar to BF II. You’d be looking at a Dow down by more than 5,000 points since Thanksgiving, declines approaching 15 percent in the S&P and Nasdaq, and a $7 trillion drop in the Wilshire.

Do you have the staying power to stick with stocks if that happens? Would you run out of money? And if you bought stocks by using money that you borrowed from your brokerage house, would you be able to meet “margin calls” demanding more collateral?

If the answer to any of these things is no, you might want to rethink some of what you’re doing.

I’m an old guy — I recently turned 77 — and I’ve learned over more than five decades of investing to keep enough cash on hand to be able to wait out market declines, of which I’ve seen plenty.

It takes a lot of self-control to hold significant amounts of cash, given that it’s earning essentially zero thanks to the Federal Reserve’s near-zero interest rate policy.

But BF II reminded me, once again, why holding cash is an important part of successful long-term investing. It should remind you of that, too.