The rule of ten best days in the market states that the S&P 500 makes most of its annual returns in just ten days which means don’t time the market. 

Since 1928, the S&P 500 has returned a CAGR return of +8% over the decades. If you exclude the 10 best days, the S&P 500 has returned -13%, according to Tom Lee, CIO and Portfolio Manager, Fundstrat Capital.

This strongly supports staying invested, and not trying to time the market.

Studies using long S&P 500 history show that if you stay fully invested, your long‑term annual return is much higher than if you miss just a handful of the very best days.

For example, one analysis from 1980–2022 found $10,000 grew to about $1.1 million if fully invested, but missing only the 10 best days cut the ending value roughly in half.

Similar work over 1990–2024 shows that missing just 15 best days lowered annualized returns from about 10.7% to 7.6%, and missing more of those days nearly erased the equity return advantage. The practical takeaway is that being out of the market at the wrong moments can be extremely costly.

For the typical retail investor, the message is: avoid emotional in‑and‑out decisions, because most people who try to time the market end up missing more good days than bad and underperform simple buy‑and‑hold.

Creating and staying with a financial strategy can help you avoid making rash moves in response to what’s happening in the market.

Source: https://www.morganstanley.com/atwork/employees/learning-center/articles/cant-time-market