10:16 a.m. EST January 4, 2015  January marks a fresh start for Wall Street, a time for investors to reassess risks and put fresh cash to work. The first month of the year is also known for setting the trading tone and mood of the market for the full year. So, will January tell a bullish or bearish story for 2015?

There is an old saying on Wall Street: "As January goes, so goes the market." The direction of the broad U.S. stock market has tracked January 77% of the time since 1950 — and "registered only seven major errors" — according to The Stock Trader's Almanac. Last year, however, was an exception, with the Standard & Poor's 500-stock index tumbling 3.6% in January before rebounding strongly and finishing the year up 11.4%. Gains in the first five days of January have led to full-year gains 85% of the time.

January's predictive nature has been even more spot-on in pre-presidential election years, with the U.S. stock market's full-year direction determined 87.5% of the time by how the market fares in January, according to Almanac editor Jeffrey Hirsch.

"Getting off to a good start is always important," says Hirsch, especially this year when the Federal Reserve is expected to start hiking interest rates for the first time since 2006. Like 2014, January 2015 could see profit-taking after another good year, he adds.

Given that the aging bull market is nearing its sixth birthday in June and with the stock market coming off its third straight year of 10%-plus days — its best three-year surge since the late 1990s — it raises the question: Is the stock market's behavior this January even more important than it has been in past years?

"It depends on what kind of behavior it is," says Woody Dorsey of Market Semiotics, an investment research firm.

Indeed, how stocks fare at the start of a new year often influences how investors perceive the market for the longer haul.

"If the stock market soars for five days and then whimpers back to unchanged by month end, it's very different than if it gradually trades up," Dorsey explains. "If it's deemed to be bullish, it feeds into the Wall Street machine of pushing the upside bias."

A weak start to January could put a chill on risk-taking and propel profit-taking by investors that fear tougher sledding ahead.

The stock market, of course, gets a natural shot of adrenaline most Januaries in the form of a fresh infusion of cash, says Quincy Krosby, market strategist at Prudential Financial.

"January sees new money coming into the market, money that needs to be allocated," Krosby explains. "The infusion of capital comes from (employee bonuses), 401(k) re-allocations, rollovers from retirees, and new employees setting up accounts. Institutional money from global pension funds and endowments, sovereign wealth funds and global mutual funds (also) has to be put to work. This capital provides an important source of liquidity for markets."

Cash freed up by investors that sold losing stocks near year-end also finds its way back into the market. The stock market also benefits from fresh optimism as an old year turns to a new one, adds Dorsey.

"Maybe people come out of the holidays with a hopeful bias," says Dorsey. "Let's face it, Wall Street has a bias for the upside. It is irrational but real."

So what will drive markets this January?

Many of the same things that were driving markets at the end of 2014, says Darrell Cronk, president of Wells Fargo Investment Institute.

"A continuation of strong economic data in the U.S.," says Cronk, citing consumer confidence at its highest level in the current recovery, still-strong manufacturing data and what historically has been a "seasonally a strong period for jobs data."

Wall Street will also likely trade on growing hopes for more stimulus measures taken by the European Central Bank, as well as anticipation of a strong start to the fourth-quarter earnings season when companies begin reporting results in mid-January.

Playing devil's advocate, what could derail markets from their current record-setting run once the calendar turns to January?

Here are four things to worry about:

1. Risks from abroad. "The risks are primarily external," says Song Won Sohn, an economics and finance professor at California State University Channel Islands. "The Eurozone could relapse into a recession. Japan could falter. The biggest risk is China whose growth is slowing significantly. China imports about $2 trillion worth of goods every year. No doubt there will be more geopolitical turmoil in Eastern Europe and the Middle East."

2. Unknown shocks. "The risks are always the unknown, and the unknown is always unknown," says Dorsey. "One characteristic of the future is that it is inherently unknown. Maybe the surprise will be the degree of volatility that is coming. I would be wary of a whipsaw in January."

3. Oil-related angst. Continued fallout from plunging oil prices could cause fresh market turbulence. "Oil price volatility without supply adjustments from global oil producers," says Cronk.

4. Interest rate fears. Krosby says the market could be upended if growth speeds up (too fast) and the Federal Reserve appears to be "behind the curve," forcing them to raise rates too quickly and sooner than Wall Street expects.