The S&P 500 index has jumped over 6% this month as of May 29. That put the index on track for its biggest monthly gain since November 2023 and potentially its best May since 1990.

Investors heeding the adage “sell in May and go away” would have missed out on big gains this month in the U.S. stock market. 

NRG Energy led the gains with around 41% rally. Constellation Energy surged 36%; Seagate Technology, Super Micro rose more than 29%; NVIDIA, Tesla Motors rose more than 27%.

The S&P 500 is seeing a strong rebound after three straight months of declines. Markets saw significant turmoil in early April after President Donald Trump’s “liberation day” tariffs sparked fears of a recession and higher inflation, but large levies rolled out by the White House have since been paused as the White House aims for progress on trade negotiations.

Investors cheered the mid-May breakthrough in talks between the U.S. and China, which resulted in lower tariffs for 90 days. Then, over Memorial Day weekend, Trump said he was delaying a massive 50% tariff on the E.U. until July 9.

The S&P 500 rallied 2% on Tuesday as U.S. investors, on their return from the three-day holiday weekend, pushed stocks higher on the E.U. tariff pause. That marked the S&P 500’s strongest gain since May 12, the day the stock market rallied on the positive outcome of the U.S.-China trade talks.

“Back when the U.S. was more of an industrialized economy, it was common for plants and factories to close for a month or longer in the summer,” the First Trust strategists said. “This downtime allowed businesses to retool and gave employees time to vacation.”

That gave way to the “sell in May” theory, which was based on the idea that “companies would conduct less commerce in that six-month span, which would likely translate into lower earnings, and temporarily reduced stock prices,” the strategists explained.

But today, the U.S. economy is “so technologically advanced and globally competitive that companies can no longer afford these extended periods of stagnation,” they wrote.