Add next appointment merge tag to your invoice
You can read how to modify an invoice template in the article: Create / edit an invoice template
When modifying the invoice template, you'll find the merge tag [Date of next appointment] under the Patient section in the list on the left-hand side. When sending the invoice, this tag will be replaced by the date of the patient's next appointment.
Add next appointment merge tag to your communication
You can read how to modify an email template in the article: Customise mail templates
When modifying the email template, choose for "Merge Tags" in the menu for your text field. Under the Patient section, choose for the merge tag [Date of next appointment]. When sending the email, this tag will be replaced by the date of the patient's next appointment.
In addition to the Merge Tag [Date of next appointment], you also have the option to choose [Treatment type of next appointment], [Therapist first name of next appointment], [Therapist last name of next appointment], and more.