But does the saying even hold true this year?
You could easily argue this year—or even the last five years—have been like no other.
This year, global markets have been whiplashed by the European Central Bank (ECB)'s quantitative easing (QE) program, the Swiss National Bank's shock decision to remove the currency peg and volatile swings in the oil price. That's not to mention the bull run in the U.S. dollar, which then petered out, only to be topped off by a fair amount of vicious moves in the bond market to take yields back to pre-QE levels and a good dose of ECB verbal intervention to take yield up again.
Read MoreWhy oil stocks aren't out of the woods
So what are the experts saying?
Bob Iaccino, chief market strategist at Tethys Partners, recently told CNBC that "Sell in May" does NOT hold this year: "Every old adage needs to be put away, because we are in an era that we haven't seen before. We don't know how it's going to be when the Federal Reserve comes out of this record liquidity, money printing—we don't know how any of the markets are going to react."
Iaccino, who is focused on U.S. markets, added: "What you're seeing is that the employment cost index is showing a dip and that lowers individuals earnings, so the retail sales numbers will be increasingly important. If you sell in May and go away, you may miss the next move, if you're not invested, you may miss the next move, but you have to have a strong stomach to be involved. "
But not everyone agrees about the advisability of staying invested in risky assets.
According to Bank of America Merrill Lynch, you might be better off abiding by the old saying this year, as their analysts are predicting a "scary summer for markets".
The bank's chief investment strategist, Michael Hartnett, writes that investors are "trapped in a twilight zone"—the uneasy transition from ultra-easy monetary policy to the gradual normalization of policy as the Fed prepares to hike rates.
During this wobbly in-between phase, investors are in for a rocky ride, including "mediocre returns, volatile trading, correlation breakdowns and flash crashes".
In a nutshell: Reduce risk during the summer, says Hartnett.
So whether to believe old market adages or not—unfortunately I cannot tell you. But with the ominous summer months approaching, the consensus seems to suggest that volatility is here to stay—so buckle up.