Highlight All Mondays in Orange Excel

You've two issues in your formula:

• your formula should refer only to one cell (the one which is active when you write your formatting)
• default day numbering of WEEKDAY function is 1 to 7, it'll never be 0 (in default format Monday is 2)

Your formula might have worked when all those days are Mondays.

In the Conditional Formatting, you need to enter the formula for one cell. It will be automatically applied to all others, relative or absolute, as requested.

So select all the cells you want it applied to, and in the decision formula, write =WEEKDAY(B1)=0 (or =WEEKDAY(B$1)=0).

To verify what I claim, you can click on for example E1, and look up the formula in the conditional formatting, and it will show =WEEKDAY(E1)=0 - as it should.