Hi Jamie:
That’s expected for day views containing no events, One thing we do – and you can see this if you view the source – is set a tag instructing search engines not to index these pages:
<meta name="robots" content="noindex,follow" />
This has been correctly set on the page you referenced. Is it possible then that the information you are getting from webmaster tools is purely diagnostic in nature, rather than suggesting there is a problem that will impact on your search ranking?
If you would prefer day views do not link to other, empty day views – or would prefer day views do not return 404 errors, we’re certainly open to your feedback if you would like to post it on UserVoice: we’d certainly be open to exploring changes for future releases.
Thanks!