S&P 500 ETF just had its third-worst open ever, behind only the financial crisis and 9/11

• Monday's open in the ETF representing the S&P 500 was 7.4% lower than Friday;'s close, the third worst opening gap ever.
• According to Bespoke, the worst days were in October 2008, during the financial crisis and Sept. 17, 2001, the day the market opened in the wake of the 9/11 terrorist attacks.
• The big opening gap signals more volatile trading ahead and a potential move by stocks into bear market territory, according to Bespoke.

A trader works on the New York Stock Exchange on March 3, 2020.

Michael Nagle | Xinhua via Getty

The widely traded ETF representing the S&P 500 had its third-worst open ever, following the financial crisis in October 2008 and in the wake of the Sept. 11, 2001, terror attacks.

The opening gap down in the SPY Sector SPDR S&P 500 ETF was about 7%, Bespoke Investment Group founder Paul Hickey said. While the market behaved differently in the aftermath of both those prior days, the message from Monday's move is that the market will continue to see volatile swings, and they could be lower.

"In one period, you saw continued weakness over the next week. The other period you saw continued weakness over the next month. It just says get used to the types of moves we've seen over the last few weeks," Hickey said.

The worst opening gap for the S&P ETF was in the thick of the financial crisis after Lehman Brothers failed and other institutions were wavering. The opening gap, or difference between the prior close on Oct. 23, 2008, and open, was 8.3% for the SPY ETF, used by individual investors and big traders as a proxy for the stock market.

The market was down 5.1% that day, and it was down 5.2% on Sept. 17, 2001, the day it reopened after 9/11. SPY was off 8.2% on the opening gap after 9/11.  The S&P 500 closed down 7.6% Monday, while SPY was down 7.8%.

A week after the 2001 opening drop, SPY was down 3.5% for the week but up 5.5% for the month. It ended down 15.8% a year later. After the Oct. 24, 2008, drop, the market was up 11.2% a week later but down 2.3% a month later.

A year later, in October 2009, the SPY was up 22.8%. That was seven months after the start of the bull market, which was launched 11 years ago Monday on March 9, 2009. The S&P 500 is now down more than 17% from its all-time highs, reached just last month. A 20% decline would indicate a bear market, and the end of the market's record-setting bull run.

"It wouldn't surprise us to hit that 20% threshold," Hickey said. "When the comparisons are 9/11 and the financial crisis, it just illustrates it."

The S&P 500 and SPY were down 5.5% in morning trading, after opening down 7.4% from Friday's close, according to FactSet.