Add the day of the week to the body classes.

functions.php

<?php

// Add the day of the week to the body classes.
add_filter( 'body_class', 'add_day_of_the_week_to_class_names' );
function add_day_of_the_week_to_class_names( $classes ) {
    $classes[] = strtolower( date( 'l' ) );
    return $classes;
}