Get all posts which was posted on X Days WordPress

You can try following code

$y = date('Y');
$m = date('m');
$d  = date('d',strtotime("-7 days")); //Date you want

$args = array(
    'posts_per_page' => -1,
    'post_type' => 'post',
    'date_query' => array(
        array(
            'year'  => $y,  
            'month' => $m,
            'day'   => $d,
        ),
    ),
);
$posts= new WP_Query( $args ); //you will get posts which are posted on specific date