S&P500 has considerably risen over the last year, and its most recent behavior has been noticeably different from its usual long-term trend. In the graph, you can see a linear regression plot that was built with data from November 2008 up until today. The upper and lower lines show the boundaries of the regression’s confidence interval at the 95% level.
As can be seen, the current price of S&P has now crossed the upper limit of the confidence interval which might signify a low probability of continuation of the positive trend (at least, in the medium- and long-term perspectives). Investing in the index right now might entail large risks, since the price is very likely to bounce back in the months to come.
Some support for this comes from the bouncing of the price in April 2020 when S&P broke the lower boundary of the confidence interval and then rose rapidly in the direction of the long-term trend.
Thus, it might be more reasonable to enter the market when the price corrects down closer to the long-term path. Such a maneuver has a chance to minimize your risk of extreme drawdowns.