Custom Post type – how to get checkbox to update meta field to null

It appears the post meta is not being cleared for ‘sunday’ when the checkbox is unchecked.

If( isset($_POST['sunday']) ){
    update_post_meta($post->ID, "sunday", $_POST["sunday"] );
}else{
    delete_post_meta($post->ID, "sunday");
}
return $post;

Or you can set the value to false

If( isset($_POST['sunday']) ){
    update_post_meta($post->ID, "sunday", true );
}else{
    update_post_meta($post->ID, "sunday", false );
}
return $post;