@Thomas-And-Susan0 If you are not using a PMS you will have to do this manually. What I would do is when I know a guest is going to be staying overnight on a Sunday, I would set up a reminder to send.
Create the reminder email and then save in your Google calendar with a prompt/reminder for you to send at noon on Sunday. It would just be a copy and paste. There are some 3rd party apps that may be able to help with this. Many email programs have a "send Later" option as well so you could create and send later as needed.
I use OwnerRez as my PMS and I have everything automated including specific messages for specific scenarios. Hope this helps 😀
Karen