Check that the date in the email actually exists.
Many times people put the day of the week and the date together in their email but they get them wrong. For example, Thursday, the 30th of June doesn't exist in either 2017 or 2018. Could you pop a window that the date entered doesn't make sense. Or maybe even tell the user that the next June 30 on a Thursday is 2021 and it would be helpful to add the year on your email for clarity sake :-)