Notifications are sent at 12:01 AM on the date Scheduled. 

For example:

A contract notification is scheduled to be sent 2 days before the due date, then on that given date at 12:01 AM, the notifications will be generated, and the emails sent shortly afterward.
Note: The only exception would be if a scheduled notification specifically included a 'time' field.
You can verify if/when most e-mail notifications were sent (and to whom) by:
  1. Navigate to Core > Communication > Notifications > Notification History
  2. Select a category and date range (Ex: Contract Reminders can be seen in the 'ReEnrollment' category)
  3. Click 'view'