The slipped slightly on Friday morning, in the index's second down day following a huge gain on Wednesday — but an interesting trading pattern may predict a late-day rally.
The S&P was also lower as of midday trading on both of the prior two Fridays. But in both sessions, the bulls fought a valiant fight into the close, leading the market to rise powerfully in the last hour of trade. As a result, the S&P closed at the dead highs on both Feb. 17 and Feb. 24.
What explains the striking moves? Perhaps it comes down to fund managers' anxiety about relative performance.
"I think this is what they're calling, nowadays, 'FOMO' — fear of missing out," Mark Tepper of Strategic Wealth Partners said Thursday on CNBC's "Trading Nation."
"Most investors, advisors, managers want to be in the market, and they're using every opportunity at a pullback to invest more and more into this market," Tepper said.
Dennis Davitt, a portfolio manager with Harvest Volatility Management, says that the market's momentum has forced the hands of many investors who would have preferred to sit on the sidelines.
"It feels like the market is underweight stock," Davitt said on "Trading Nation." Yet since old levels of resistance have become levels at which "there's a tremendous amount of support, if the market does sell off, people need to buy stocks going into the end of the day."
Matt Maley, equity strategist at Miller Tabak, put it in even starker terms.
"People who think the world's coming to an end are still getting more long because they have no choice," Maley told CNBC in a Friday phone interview. "Everybody's scared to death about their performance."
Still, he points out that last week's end-of-week rally came just a few days before the end of the month. The rise, then, may have reflected short-term performance fears related to lagging further behind if investors became yet more optimistic over the weekend.
For that reason, "I don't think we'll necessarily get that [late-day rally] today," Maley said.
Of course, only time will tell.