You created a reminder for this event.
OR
Another user created a reminder and added you as a CC for this reminder.
Disabling This Message
If you created the reminder, delete it by going to the calendar, clicking on the event to view the event details, and then click on the Alarm Icon.
If another user created the reminder, contact them and ask to be removed from the reminder. The contact information for the person who created the reminder will be displayed in the Additional Information section.