Hey there, Beth!
I was able to view the next month’s calendar using Safari on Mac OSX. Plus, I don’t see any obvious conflicts when checking out your site. That’s all good news!
I wonder if the visitor who reported the issue is on an older version of Safari. There was a similar thread to this last month and it sounds like the issue was related to an outdated/unsupported version of Safari.
One way you could solve this on your end is to disable the AJAX used to navigate between months. This answer in the same thread outlines how that works.
Will that work for you? Let me know if that helps.
Cheers!
Geoff