All fields of type date must have a day specified. That's because the date is stored internally as an integer recording the number of days from 12/31/0000 to the date you entered. Without a day, there's not enough info for FileMaker to compute that value.
What you can do, is set up the OnObjectValidate trigger to run a script that checks the data in this field before FileMaker can try to validate it as a date. If a day has not been specified, the script can modify the text entered to supply a default day such as 1 to create a date that won't trip this validation error.
If I did that wouldn't the script then add a day to make it valid then? Or would the script just be tricking the validation system letting it believe that there was a day while the field reflected just the month and year? If so what would the script look like?
Yes, it adds a day to the date. As I stated in my last post:
the script can modify the text entered to supply a default day such as 1 to create a date that won't trip this validation error.