U.S. stock indexes booked their worst trading session on the eve of Christmas in history in a holiday-shortened session, putting the S&P 500 on the brink of the 20% decline from a recent peak that is commonly considered a bear market.

Wall Street trading on the New York Stock Exchange closed at 1 p.m. Eastern Time, on Christmas Eve and will be closed on Tuesday for the holiday.

The Dow Jones Industrial Average DJIA, +0.19% finished 653.17 points, or 2.9%, lower at 21,0792.20, marking its lowest close since Sept. 7, 2017, while the S&P 500 index SPX, +0.29% fell 65.52 points, or 2.7%, to 2,351.10, representing its lowest since April 21, 2017.

Meanwhile, the Nasdaq Composite Index COMP, +0.36% COMP, +0.36% traded down 140.08 points, or 2.2%, to end at 6,192.92, marking its lowest close since July 10, 2017.

Read: Opinion: Look for S&P 500 to bottom between 2,250 and 2,335

All three indexes have fallen for four straight sessions. The Nasdaq has skidded 8.7% in that time, while the Dow has lost 8% and the S&P 7.7%. The trio is on track for the ugliest monthly drop since October of 2008, during the heart of the 2008-09 financial crisis. Last week was the worst week of trading since the financial crisis of 2008.

The Nasdaq officially entered bear-market territory on Friday, and has now plunged 23.6% from its record close set Aug. 29. The S&P has slumped 19.8% from its Sept. 20 record close, and the Dow has slid 18.8% from its Oct. 3 record close.

Read:As stocks suffer a December rout, Wall Street strategists bet on 2019 gains

What’s driving the market?

A fresh source of anxiety was a tweet from Treasury Secretary Steven Mnuchin that he had spoken with the CEOs of the country’s six biggest banks. to assess the health of the banking system. That raised some questions about liquidity among those institutions, a concern that previously hadn’t existed. Treasury officials insist that the calls to bank executives was just a routine checkup.

Mnuchin on Monday held a call with the president’s Working Group on financial markets “to discuss coordination efforts to assure normal market operations.” The Treasury chief is on vacation in Cabo San Lucas, Mexico.

Read: Tim Mullaney says Mnuchin can’t stand up to his boss — and it’s costing you money

Nonetheless, the tweet overshadowed any positive impulses from China’s Ministry of Commerce declaring that “new progress” had been reached in “vice-ministerial-level” trade discussions with the U.S. in a teleconference Friday, the South China Morning Post reported on Sunday.

Read:This brutal stock-market rout mirrors the 1987 crash in one important way

Meanwhile, White House advisers over the weekend tried to ease investor concerns that President Donald Trump could fire Federal Reserve Chairman Jerome Powell amid anger over the Fed’s recent interest-rate hike that spurred more market losses. Mick Mulvaney, the president’s incoming chief of staff, told ABC’s “This Week” on Sunday that Trump “now realizes” he cannot remove Powell, while Mnuchin stated Saturday that the president had not suggested firing Powell.

Read:Why Trump can’t fire Powell for disagreement over monetary policy

Meanwhile, parts of the federal government shut down Friday for the third time this year as lawmakers in Washington continued to wrangle over funding for Trump’s proposed border wall. The shutdown could last into 2019, when the new Congress returns from a holiday break.

Equity and bond markets finished early to mark Christmas Eve and will be closed on Tuesday for the Christmas holiday. The rest of the week will see normal trading hours. However, with many traders out for the holidays, volumes are expected to be thinner and moves could be exaggerated.

Which stocks are in focus?

Shares of Bank of America BAC, +0.34% , Citi C, +0.81% , Goldman Sachs GS, +4.83% , JPMorgan Chase JPM, -0.08% , Morgan Stanley MS, +0.40% and Wells Fargo WFC, +2.14% ended the day solidly lower in the wake of the concerns raised by Mnuchin’s tweet.

Apple AAPL, +1.02% sank 2.6% to close at the lowest level since July 12, 2017, and its market cap fell below $700 billion for the first time since Feb. 10, 2017.

Read:Billionaire David Tepper says stock market tumbled too sharply Monday — so now he’s ‘nibbling’

How are other markets trading?

Asian stocks had a mixed session, with China’s Shanghai Composite SHCOMP, -0.35% modestly higher, while the Nikkei NIK, +0.49% was closed for a holiday. European stocks SXXP, -1.02% pointed to a weaker start.

The ICE Dollar Index DXY, -0.02% was down 0.5% to 96.49, while gold US:GCG9 jumped 1.1% to settle at $1,271.80 an ounce. February West Texas Intermediate crude US:CLG9 rose fell below $44 a barrel and was extending its decline toward a fresh 17-month nadir.

What are strategists saying?

“Stocks remain trending strongly downward in a trend that’s nearly erased 50% of the entire rally from 2016 in S&P,” wrote Mark Newton, independent technical analyst at Newton Advisors, in a Monday research note.

“Last week’s ‘Make-Or-Break’ comment was decided by the nasty ‘Break’ on Monday which resulted in severe acceleration and resulted in the worst week of performance in a decade. The decline has begun to take a toll on the broader structures, which now show long-term trend breaks on monthly logarithmic charts from the 2009 lows,” Newton wrote.