Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘greenday_head’ not found or invalid function name

I would start by creating in the themes functions.php a function like this

public function greenday_head($args) {
  return $args
}

It may be that $args are never passed in but it seems likely they are. If you want to try to find out what this is doing then you can add:

public function greenday_head($args) {
  echo '<pre>' . var_export($args, true) . '</pre>';
  echo '<pre>' . var_dump(debug_backtrace()) . '</pre>';
  return $args
}