Hi Patrick,
Thank you for providing this info – it helped to dig a little deeper into your issue.
From what we now see from your snippet and site there may also be a JS error involved. Your symptoms pointed be to what i though was the body class issue – but now I can see the snippet you provided, the theme is actually treating that correctly.
A step that I forgot to get you to take at the start, which I should have done right off the bat, was to get you to check if there is anything else on your install causing a conflict. I would start with all your plugins by disabling them and switching them back on one at a time. If none of that works then I’d try to switch to a default theme too and see how the calendar works.
If you find anything here then I think we may be able to narrow it down to the offending issue.
Neill
– Neill