add_action( 'woocommerce_process_shop_order_meta', 'woocommerce_process_shop_order', 10, 2 );
function woocommerce_process_shop_order ( $order_id ) {

 //Get this order id dynamically
 $order = wc_get_order( $order_id );

 // The text for the note
 $note = __("Custom Order Note Here");
 $note_date = date('d.m.Y',strtotime("-1 days"));

 // Add the note
 $order->add_order_note( $note );
 $order->wp_insert_comment($note_date);

 // Save the data
 $order->save();

}