The day before turning in its fifth-worst day in history—its worst single-day performance in more than three decades—on Friday the S&P 500 turned around to record its tenth-best daily return in history. That it ended on a strongly positive note may have been little relief for the anxious, as the index remains down just under 20% from its February record peak. The two-way volatility we saw last week offered a strong reminder that remaining invested during times of market tumult generally has proved the better course for those with reasonably long investment time horizons.