add_action( 'pre_get_posts', 'danniel_show_future_posts' );

function danniel_show_future_posts( $query ) {
    global $wp_the_query;

    // Main query only and on a category archive
    if ( $query === $wp_the_query && $query->is_category() ) {
        $query->set( 'post_status', array( 'published', 'future' ) );
    }
}