Update 27 March 2020
I re-wrote the underlying code to allow for a more generic set of parameters. you can now copy the sheet to your own version and change the ticker, the # of days of history plus whether or not to include Fridays in the worst day calculations. Have fun!
Background on what defines a Really Bad Day (RBD) can be found here. It contains the most succinct reference to livesoft's rules that I have found, but be warned there are many many variations. The true definition of an RBD is probably "I'll know it when I see it" (see Jacobellis v. Ohio). I am sticking to this one because it has the key components stated by livesoft in a form that are mostly easy to algorithmically analyze.
A livesoft-defined RBD meets all of the following criteria:
An S&P 500 Index fund drop of 2.5% or more
A drop greater than the 4th largest drop within the previous 150 days
Not a Friday
To make this calculable, I have modified these rules very slightly to be more precise.
Has the S&P 500 Index tracker as specified by googlefinance("INDEXSP:.INX") dropped more than 2.5% today?
Is this drop greater in magnitude than the 4th largest drop in the last 150 calendar days?
I assume that the drop calculation does not include Fridays.
3.) Is date in question NOT a Friday?
If all three are True, then an RBD Alert is shown.
I give two slightly different Readings of rules #1 and #2
The lowest point during the trading day so far
The current index value
You can decide which metric you want to use as I present them side-by-side.