add_filter('woocommerce_checkout_fields','date_field_classic_checkout_validation');

function date_field_classic_checkout_validation($fields) {
	 $fields['billing']['date'] = array(
        'type' => 'date',
        'label' => 'Delivery date',
        'priority' => 151,
        'required' => false,
        'validate' => array('date'),
        );
	return $fields;
}