Update: A 'Santa Rally' for stock markets has been knocked off course after US president Donald Trump stuck to a hard line on tariffs with China and bond markets stoked fears of a US recession.
As a mixed year for markets draws to a close, bullish investors had been on the front foot, buoyed by two pieces of news.
First, Federal Reserve chair Jerome Powell sparked a jump in stock markets after his claim that interest rates were 'just below' neutral appeared to suggest the US was nearing then end of its interest rate hiking cycle.
That rally then accelerated on Monday after the US announced that it had reached a 90-day truce with China over trade, agreeing to halt additional tariffs.
But Trump has undone that optimism with a series of tweets yesterday in which he emphaised his intention to ratchet up the pressure on tariffs if the talks with China fall through.
The downward lurch for stock markets threatens to buck the tradition of good cheer over the Christmas period.
There is strong evidence for the trend. Online stockbroker Bestinvest found that since 1979, global markets have risen 77% of the time in December, with a median return of 1.3%.
Similar analysis from fund group Schroders found global stock markets' December returns were well ahead of those for other months of the year.
The trend is even more pronounced for the UK stock market, with the FTSE 100 rising 79% of the time in all the Decembers since 1979, with a median return of 2.4%, according to Bestinvest.
Figures from fellow stockbroker Hargreaves Lansdown and fund group Fidelity meanwhile show the UK stock market returns over individual years.
Fidelity found that over the last 30 years the FTSE 100 has risen in 26 Decembers. The only falls came in 1994, 2002, 2014 and 2015, with the heaviest 2002's 5.4% loss.
Hargreaves found a similar pattern for the FTSE All-Share, which has risen in 27 of 32 Decembers since its launch in 1986, delivering an average return of 2.6%. Over all months over the period, the FTSE All-Share rose only 62% of the time, and by an average of just 0.7%.
The FTSE All-Share's heaviest December loss, of 5.3% in 2002, is meanwhile the lowest maximum drawdown of any month over the period.
'The data suggests that the so-called Santa Rally is a very convincing phenomenon,' said Jason Hollands, Bestinvest managing director.
'What is less clear is why markets have a tendency to rise during the final month of the year. One theory is that year end positive momentum in the markets may be down to fund managers reducing their cash weightings and “window dressing” their portfolios with stocks that have performed well ahead of reporting periods to clients in the New Year.'
Stock markets may have started the month in buoyant mood, but crucial to that optimism persisting will be the outcome of three key events over the next few weeks.
On Thursday the Opec cartel of oil producing nations will meet against the backdrop of an oil price that has tumbled from over $86 a barrel at the beginning of October to $62 today.
That's up from a low of $58 at the end of November, as expectations of a cut to production levels have begun to build. Stock markets could do without any major shocks from the oil market this week to preserve hopes of a Santa Rally.
Next Tuesday MPs will vote on prime minister Theresa May's Brexit deal, with the outcome likely to have a major impact on the UK stock market and the pound.
'Market participants appear to be giving little chance of Mrs May winning the parliamentary vote on 11 December,' they said.
'A defeat in the Commons may not lead to the much heralded "market chaos" since the outcome is well anticipated and the consensus amongst investors is that some kind of amendment is possible.'
Lastly, the Federal Reserve will on 19 December deliver its interest rates decision. Another hike is widely expected but of more importance to investors will be the tone of the Fed's comments.
'Having provided the upward impetus for markets since the financial crisis, central banks are rapidly becoming one of the biggest threats for investors, both in trying to move towards higher interest rates and via the removal of quantitative easing,' said Rebecca O'Keeffe, head of investment at Interactive Investor.
'The Fed is having to play a Goldilocks role in making sure that it maintains a middle ground of not too hot, not too cold, whilst at the same time trying to wean investors off low interest rates and quantitative easing.'