Downtime Summary report is like a system generated report and is triggered on a daily basis. We do not have option to edit any attributes of this report.
After reading through your post following is my suggestion:
You can go to the server monitor details page. Click on 30 days Icon Under Today's Availability and you can add reason for downtime for any monitor. You can then schedule a Downtime History report. From Reports Tab-->Click on schedule reports link on the top right-->Select Report Type as Downtime History report and schedule it. This should give you the details you are looking for.
Thanks & Regards, Paul Jacob
Leave a comment on paulpjacob's reply
Change topic type
Link this topic
Provide the permalink of a topic that is related to this topic