Would like to send an email when a date field is populated

I have a single request table with a date field of "Date_Completed" and I have an edit page that staff will use to fill in the completed date of the record once it has been acted on. What I would like to do is send the customer an email that lets them know we have taken care of their request when the completed date field is populated. I think I need to add script to the Events section but unsure of the code and on which event would be the best trigger. I can use the "Send email with new data" add action but I need some help with the If statement on the date field.

Thank you in advance for any suggestions