CNBC's Jim Cramer warned investors on Monday that positive stock market openings could fade during the trading day, especially during September, which has notoriously been a tough month. "I'm concerned that we have another positive opening and people come in and buy and then they get hurt. So I really caution people to come in," Cramer said on "Squawk Box." In fact, he said Wall Street should pay particular attention to Sept. 17 — this Friday, which also happens to be a "quadruple witching" day of expiring options and futures on indexes and individual stocks. "September 17th has been historically for the last 20 years the beginning of a major downturn. And we're right on the eve of that," Cramer said, reiterating trends seen in technical analysis from longtime chart analyst Larry Williams. The Dow Jones Industrial Average rose more than 300 points in early trading Monday, rebounding from five straight down days. The S & P 500 was also higher, after falling five sessions in a row. Both indexes were still up in early afternoon trading. On CNBC's "Mad Money" on Friday , following sharp slides in all three stock benchmarks, Cramer said, "Williams points out that the optimal seasonal sell point continues to be the same, around September 17th." This is when prices "tend to experience a precipitous drop," Cramer added. "If history's any guide, there's a high probability this market will get hit with a meaningful decline at the end of this month, which is one reason, I think, it's been so soggy for the last week," Cramer also said Friday, following weekly drops of almost 2.2% and 1.7%, respectively, for the Dow and S & P 500. Those were their worst weeks since June. The Nasdaq posted its worst week since July, falling 1.6%. Even when September ends, investors often are not out of the woods on a seasonal basis. October has also been a rough month for the stock market. But after that and until the end of the year, Wall Street has historically gotten a boost — particularly in December for a period known as the Santa Claus rally. — CNBC's Kevin Stankiewicz contributed to this report.
A view of the New York Stock Exchange Building on Wall Street.
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CNBC's Jim Cramer warned investors on Monday that positive stock market openings could fade during the trading day, especially during September, which has notoriously been a tough month.