Please see the demo of the issue here: https://jsfiddle.net/ea8utdyx/

Here are the steps to reproduce the problem:

1. Use a nonstandard date format, like 'D M d yy'
2. Pass dayNamesShort, example: dayNamesShort: ["Su", "Mo", "Tu", "We", "Th", "Fr", "Sa"],
3. Pass a callback function to beforeShowDay argument

Notice that the date which is passed to the beforeShowDay callback is incorrect, it's about 6 years in the future. As a result, we cannot determine if a date should be visible or not.

In the demo, I have added console.log, which in the callback, you can check that it doesn't log the correct date.

I notice the problem doesn't happen when we comment the dayNamesShort argument.