Invalid Date in Archives

I’ve got an odd date showing up in my Archives: December 9
There are no posts associated with this, and clicking on it brings up a 404 error.

When in Admin, December 9 shows up as a date filter for my posts as well, but once again, selecting it brings up a 404.

What is going on here?, and how can I get rid of it? I have no direct access to the DB to remove anything manually.

http://centerville.usswim.net/