If you'd like to set up booking reminders sending time, please refer to the following steps and modify in backhouse :
Login Backhouse > Company Settings > Click Edit
Click App Features > Store/Branch Settings > select the branch > Time of day to send booking reminders
Number of days in advance to send booking reminders key in how many days before seated you want to send the booking reminders
Time of day to send booking reminders key in the timing of sending booking reminders
Hit Save after all set up done