Missing Good and Bad days
Motivated by a discussion on Bogleheads

Once upon a time we talked about the effect (on your Portfolio) of missing the "best" months or "worst" months. Click!
Now we'll consider missing the days which gained 5% or more and missing days which lost 5% or more.
>Just 5%? What about 4% or ...?
Suppose we'd invested in the S&P500 for the past ten years.
Suppose, too, that we made no gains on those days when the daily return exceeded 5%.
Our portfolio would look this
Note the effect of missing days when the daily return was less than -5%.

>Looks like it matters more to get those good days.
Yes, for that period and that asset, you wouldn't want to have missed the +5% days.

>And what about 4% days? And what about some other asset? And what about ...?
There's a spreadsheet to play with where you can change the 5% to something else ... and, of course, the stock.

Here are some examples where you miss +5% days or -5% days:

Here are some examples where you miss +4% days or -4% days:

Here are some examples where ...

>Where's the spreadsheet?
Here. Just click on the picture:

>It'd be neat if I could see what missing months might ...?
That spreadsheet also has a "Missing Months" sheet that looks like so (starting with a $10 investment in the DOW):

Unlike the previous sheet, you can't pick the asset on the "Months" sheet.
It has just the monthly returns for the DOW since 1928.

>Speaking of months, I understand that September is the worstest month, right?
For the DOW, yes.
Everybuddy remembers the worst day was in October (1929).
But the worst month was (gasp!) in September (1931).
Indeed, the average return is also worst for September:

>Uh ... isn't this the month of September??
Don't remind me!