function events_only_in_home_loop( $query ) {

 if ( is_home() && $query->is_main_query() ) // make sure to only run on home page
    $query = array(
        'post_type' => 'ai1ec_event', // select only event post type
        'orderby' => 'meta_value_num', // order by a meta key
        'meta_key' => 'start', // order by event start
        'meta_value' => date('Ymd'), // order by event start
        'meta_compare' => '>='
    ); // order by event start
 return $query;
 }

 add_filter( 'pre_get_posts', 'events_only_in_home_loop' );