I really appreciate your response. Besides getting my expression to work, it was very helpful and caused me to go back and try to get a better understanding of it.
Is the MAX function needed only when using this expression in a table? I tried experminting with the MAX function in and out of the variable and it also seemed to work when MAX was removed. However, if no MonthNumber was selected, it returned 0. I needed to make a MonthNumber number selection to have it return a number.