When medieval travelers needed lodging for the night, they went looking for a harbinger. As long ago as the 12th century, harbinger was used to mean "one who provides lodging" or "a host," but that meaning is now obsolete. Later on, harbinger was also being used for a person sent ahead of a main party to seek lodgings, often for royalty or a campaigning army, but that old sense has largely been left in the past, too. Those sent ahead would announce the approach of who was following behind, and that's how our modern sense of harbinger (from the Anglo-French herberge, meaning "lodgings") acquired the sense with which we are familiar today, that of something which foretells a future event.