As far as I know, this cannot be done in Salesforce reports.
In general, Salesforce reporting isn't good at showing you what is missing. I have a similar report. Even though I give it a range of 9/1/2014 to 10/6/2014, I don't get a column for each day. The report only shows me columns where there is at least one date present for the field selected as the column.
edited to add
I just remembered another project I worked on that applies here. Making sure attendance gets entered is something my organization spends a lot of time on. We track attendance in Salesforce. How can we report on what is not there when Salesforce isn't good at that? We used to export a matrix report to Excel. This had limitations; the foremost was that the results were not in Salesforce.
Our "in-Salesforce" solution is to run scheduled jobs that check each site for each open day. Where there's missing attendance, a record for the site/day is created (custom object). Now we can report affirmatively on what is missing. A second scheduled job sets the missing attendance records to "resolved" status if the attendance has been entered. This isn't simple, but it is a sufficiently important issue to us that it was well worth doing,