Meta box data is saved but NOT displayed in the meta box text field. Why?

Try this..

function custom_meta_box_markup() {
    global $post;
    $meta_box_text = get_post_meta( $post->ID, 'meta-box-text', true );
    ?>
        <div>
            <label for="meta-box-text">Day</label>
            <input name="meta-box-text" type="text" value="<?php echo $meta_box_text;?>">
        </div>
<?php }

You are missing the $post global variable which holds the post data.