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.