If A42 is the first of the month is the test, then:
=DAY(A42)=1
would be the formula.
I hope you have a color in that cell, unless you are planning on the
text not being visible.

On May 2, 1:50 pm, Amber wrote:
I have a column filled with sequential dates. Cell A41 will always be the
30th day of a month, cell A42 will be EITHER the 31st day of that month OR
the first day of the next month. IF A42 is the first day of a month I would
like to format it with white text. I understand how to use conditional
formatting (change text color) but what should the condition state? I tried;
"If the cell value is less than =DAY(31)" but that's not right.