There is an add-on called "Send Later" which will let you schedule an e-mail to be sent "later".

Thunderbird menu: Tools: Add-ons In the search box on the top left, enter "Send Later" and press the Enter key on your keyboard. For the search result showing "Send Later" click the "Install" button for it. It will tell you that you need to restart Thunderbird to finish the install. Do so. When Thunderbird has restarted, go back to the Add-ons manager window, click "Extensions" on the left side, then the "Options" button beside "Send Later". Set the options to what you want.

When you are going to send an e-mail that you want to "Send Later", instead of clicking "Send", hold down both the Ctrl key and Shift key, and press "Enter". Or click "File" in the menu, then "Send Later". This will bring up the "Send Later" options for that e-mail.

OR you can use the Calendar built into Thunderbird to set a reminder.

Thank you. That works!!