Below is a great chart from Putnam, which shows that over the last 15 years, if you missed the top 10 best days of market gains, your return would have gone from 10.66% to only 5.05%.
So, if there are 252 trading days in a year, roughly half of your return came in just 10 out of a total of 3,780 days. Improbable odds at just a 0.02% chance to guess which days those would be.
Even more significant, is if you missed the 30 best days, you would have a negative -1.18% return.