I would probably just drop a descriptive text field right beside it. So basically have my date field, and then beside it a short line that says "Creative ads are due May 15."
Ok. Thank you for the response. I was hoping for something a little more obvious. The descriptive field is only visible if a user hovers or clicks over the question mark. Also, I want to use one custom form for the entire year. I don't want to have to update the form each month.
You can read about the difference between “descriptive text” fields and “instructions” using the link above.
Regardless, there isn’t a great way to handle content that needs to constantly change, before the form is submitted.