Our ancestors didn’t need a calendar to tell them Samhain was getting close – they could read the signs. People have been reading signs in the wider world for at least as long as we’ve been human. This requires no great psychic skills – anyone can do it, if you pay close attention over a long period of time. Many things that happen regularly happen in a particular order, and while correlation is not causation it’s good enough for predictive purposes.
We’ve lost much of the wisdom and lore of our pre-Christian ancestors, and in any case we live in a very different world. But even in here in urban and suburban North America, there are signs that let us know it’s getting close to Samhain.