Getting the Most Recent Posts from Multiple Categories

I use the following for multiple post types, just replace foo and bar with your own post types:

$args = array(
            'post_type' => array( 'foo', 'bar' ),
            'orderby'         => 'post_date',
            // other parameters as needed
            );

    $q = new WP_Query( $args );

    if ($q->have_posts()) : while ($q->have_posts()) : $q->the_post(); 
    // do stuff

    endwhile;
endif;