PHP notice

Hello,

I’m getting this notice sometimes but everything still works fine.
Just FYI and what is worth:

PHP Strict Standards: call_user_func_array() expects parameter 1 to be a valid callback, non-static method the_holiday_calendar::manage_wp_posts_pre_get_posts() should not be called statically in /xxx/wp-includes/class-wp-hook.php on line 298

Thanks!