What I had tried before was to create a managed schedule through SSRS, scheduling it for the last Friday of the month. Then I went to the SQL Server Agent and modified it to run on the last day of the month. When I did this before it didn't work and caused a fatal error within SSRS.

What I discovered is that you also have to modify the the next run time column in the SSRS table to make it work. Specifically, you need to modify the dbo.Schedule.NextRunTime column to really be the next run time,

Example:

On 10/31/2008, I created a shared schedule to run on the last Friday of the month at 10pm in SSRS, I changed the start date of the schedule to 11/1/2008, which made the next run time 11/28/2008 @ 10pm. I then went into the SQL Server Agent job that SSRS subsequently created and modified the job to run on the last day of the month (now the next run time listed in SQL Server Agent is 11/30/2008 @ 10pm).

I then went into the ReportServer database on my SSRS server and modified the dbo.Schedule.NextRunTime column for this schedule from 11/28/2008 @ 10pm to 11/30/2008 @ 10pm. This now matches what's in the SQL Server Agent job and it passes the consistency check that SSRS runs.

Hope this helps.