How to fix active-class problem in minicalendar with calendar / exposed filters
Try next, that work without using urls, using second argument: 

function YOURTHEME_preprocess_calendar_datebox(&amp;$vars) {
  $arg = arg();

  // If no second argument provided - that's mean today. 
  // Please check how argument date supplied for next day: 2014-12-01 or 2014-12-1 and change date() to appropriate.
  $url_date = empty($arg[1]) ? date('Y-m-d') : $arg[1]; 

  if ($vars['date'] == $url_date) {
    $classes = explode(' ', $vars['class']); // Extract the current classes as a list.
    $classes[] = 'active'; // Add 'active' to the class list.
    $vars['class'] = implode(' ', $classes); // Concatenate and attach the updated class list.
  }
}