U.S. stocks began the week on a firm footing, but Citadel Securities is warning investors not to overlook a seasonal rough patch for equities.

Scott Rubner, Citadel’s head of equity and equity derivatives strategy, wrote in a recent note that the 39th trading week of the year — which began Monday — has historically been the weakest for the S&P 500, based on data going back to 1990. It is also the week most prone to 1% daily drops, he said.

The S&P 500 has so far gained about 3% in September, but over the past five years, September has averaged a 4.2% decline. Rubner added that the “week after next” has also tended to bring volatility, particularly in the days following September’s options expiry.

The warning underscores how seasonal factors remain a headwind, even as markets push higher in early September trade.

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