Conditional highlighting

There are many approaches to this. The specifics, though, will depend on your format.

It isn't possible to discern from your post what your spreadsheet looks like, specifically if you're using date or numeric fields.

Assuming, though, you have some field that includes the month name in cell A1, you can extract the last day of that month using the formula:

=DAY(EOMONTH($A$1,0))

where EOMONTH() is a function that calculates the last day of the month, and DAY() returns the day component.

If you want, you can hide this cell so that you don't see the number, if that bothers you.

Now you have a cell that calculates how many days this month should have, and can be used as the conditional field in your conditional highlighting. For example, if the DAY(EOMONTH()) function is in B2, set your conditional highlighting to:

Now any time the cell value exceeds the number of days in the month, the color will change accordingly.

I recommend poking around in the 'Calendar' template (at File > New) to see how it does this. Here I've unhidden the rows in the 'Month and Year' table so you can see some of the inner workings.

It's well worth exploring other parts of the template. It's full of tricks, for example in the little previous and next months upper right, where it uses Conditional Highlighting not just to fade, but to hide completely, days that are not in the relevant months.

SG