"When you get a nice round number on an index, it takes a few instances to break through it conclusively," Valeri said. "We still think we'll tack on a few percent before the end of the year."
The S&P enters the final day of August with a 3.4 percent gain, which would give it the best August performance in 14 years and about a 5 percent gain since the start of July. Markets are closed Monday for the Labor Day holiday and reopen Tuesday.
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There are several economic reports expected Friday, including personal income and spending at 8:30 a.m.; Chicago PMI at 9:45 a.m., and consumer sentiment at 9:55 a.m.
September has the reputation of being a volatile month, more likely to see a decline in stocks than gain. But in the last five years, the S&P 500 has been positive four times. When looking at Septembers that follow a strong August, the odds, however, are against another positive month. There have been 22 times that the S&P has risen more than 3 percent in August since 1928, and in three quarters of those cases, stocks were lower in September.
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The American Association of Individual Investors' bullish index rose above 50 this week for the first time since last December. The measure went to 51.92 and Bespoke notes that spikes above 50 have been rare since the start of the bull market in 2009.
Bespoke also notes that bullishness does not necessarily indicate a lower market. In the next week, the S&P was up six of seven times for an average gain of 0.8 percent. Over the next month, the S&P was higher in every case for an average 1.2 percent gain, and was more than 5.2 percent higher after a year.