Hi Ellen
It's configurable. Inside the plugin System - Events Booking Reminder, there is a parameter called Time Ranges. You can change that parameter to change the time reminder emails are sent. See the description of the parameter and let us know if you have any questions:
If you want to send reminder emails to registrants within certain time ranges, use this parameter. For example, set it to 5,14 will make the system only send email reminder from 5am to 2pm. Set it to 8,10;14:18 will allow sending reminders between 8 to 10 am and 2pm to 6pm.
Regards,
Tuan