Referrers data is stored differently now. Instead of Monday.xml, Tuesday.xml, etc., the data is stored in Xml files with the date in the filename. I believe the format is 10.02.08 (for Feb 8 2010).
One advantage of this new format is that you can store referrers data for more than 7 days now. It's a new option on the Referrers tab.
The code is trying to delete your old file, Friday.xml, but is unable to. This is probably because BE doesn't have Delete permissions on that folder.
I don't believe you'll be able to use the old referrers data in those old files (Friday.xml, Monday.xml, etc).
I would just manually delete those files from the App_Data\log folder.
I would also try and make sure the ASPNET / Network Service account has Deletion permissions on the App_Data folder. Because going forward, these new files will get deleted after the number of log "retention" days you set has passed (this
is the new option I mentioned above). And if it can't delete the files, then you'll probably get this error again.