title: jquery ui datepicker error on beforeShowDay
body: <p>I'm trying to implement a event calendar using jquery-ui's datepicker. I've successfully installed the datepicker and it shows up and working. the problem appears when I try to register the <code>beforeShowDay</code> handler. Like this:</p>
  
  <pre><code>$('#datePicker').datepicker({ beforeShowDay : function(){ }} );
  </code></pre>
  
  <p>I receive the following error in jquery-ui -file: <code>ba is undefined</code>.
  when I output something int the function, the first four times it outputs but then I receive the error.</p>
  
  <pre><code>$('#datePicker').datepicker({ beforeShowDay : function(){ console.log('test') }} );
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>// test
  // test
  // test
  // test
  // ba is undefined
  </code></pre>
  
  <p>if anyone has a clue what's the problem, please help.</p>