add_action('pre_get_posts', 'my_pre_get_posts');
function my_pre_get_posts( $query )
{
// validate
if( is_admin() )
{
    return $query;
}

// allow the url to alter the query
// eg: http://www.website.com/event/?event_season=winter
if( isset($_GET['event_season']) )
{
    $query->set('meta_key', 'event_season');
    $query->set('meta_value', $_GET['event_season']);
}

// always return
return $query;

}