It's always sketchy to make a big deal about a single trading session, especially in relation to drawing conclusions about future returns. Markets are too noisy, and sometimes there is a market event that throws things out of whack for a day or two.
At the risk of that, let's make a big deal about Monday.
Despite the S&P 500 closing within 0.1% of a record high on Monday, fewer than 35% of securities on the NYSE managed to close above their closing price from Friday. That was the weakest Up Issues Ratio since 1946.
Since 1928, when the S&P 500 was within 0.25% of a 52-week high, an average of 58% of securities on the NYSE advanced for the day. There have been 3,331 days when the S&P was that close to a high, and the histogram below shows just how unusual Monday's session was.