@Elisabetta419 When you send it depends on the message. If there are check out instructions you would like to emphasize, or you find that guests aren’t reading and following the instructions you have posted at your listing, then sending the message the night before is a good idea.
I send mine near checkout time as it’s simply a ‘thanks again for staying, safe travels onward’ type message, since my checkout list is easy to find and is super short.