For free events, the registration confirmation email is sent the moment the contact or administrator completes the registration.
For events with a registration fee, the sending of the confirmation email depends on whether the registration was completed by the contact or by an administrator. If the registration was completed by the contact, then the registration confirmation email is sent after the event registration fee is fully paid.
If the registration was completed manually by an administrator then the confirmation email is sent immediately after the administrator clicks the Confirm without invoice option, or after registration is paid if the administrator clicks the Generate invoice option.