Historically, the final week of September has been brutal for US stocks. According to market seasonality data, it’s often the weakest stretch of the year. A time when portfolio rebalancing, tax-loss harvesting, and macro uncertainty collide to drag markets lower.

But this year? Wall Street didn’t get the memo.

The Dow Jones rose 66 points, the S&P 500 added 0.44%, and the Nasdaq surged 0.7%, helped along by renewed excitement in the AI space thanks to a fresh partnership between Nvidia and OpenAI .

The market’s early stumble came after headlines about a possible shutdown on September 30, which would kick in if Republicans and Democrats fail to agree on funding. But that negativity faded as tech optimism returned, proving once again that AI remains a powerful narrative force for investors.

It was down early but made a major turnaround. It has really has excited the US stock market, naturally.

A week that should be worse

Data from equity analysts and calendar trend watchers often pin this week, the last full trading week of September, as the worst week for stocks all year. In fact, since 1945, the S&P 500 has posted negative returns roughly 60% of the time during this week, with average declines of more than 1%.

In 2022, for example, the S&P 500 fell nearly 5% across this same stretch, the worst performance in months, driven by fears of an aggressive Fed and rising bond yields. In 2015, the same week saw a decline of more than 2.5%, amid global growth fears and a collapsing Chinese stock market.

So far in 2025, though, markets seem to be holding up, for now.

But there’s still plenty that could rattle investors

But I'm still cautious, especially due to key data due later in the week, including economic growth figures and the PCE price index, the Fed’s preferred inflation measure .

Back home, attention also turns to local inflation data and retail trade numbers, which could inform where the RBA goes next on interest rates.

So while the calendar says “danger ahead,” and history urges caution, Wall Street seems to be moving on its own timeline this year, buoyed by AI optimism and resilience in the tech-heavy Nasdaq.

But don’t get too comfortable. September might still have a sting in its tail.