The feast of Samhain, or Summer’s End, is the most important in the Celtic Calendar. The shadows become longer. Winter is at hand.

On this day the heart fires are extinguished for a night, showing that even Brigid, the all-powerful goddess of hearth and home, must bow to the change of the seasons. The world of the dead and the world of the living are at their closest, meaning that if the recently departed have a reason for payback you’d better watch out. Perhaps wearing a disguise might keep unfriendlies away. Bonfires are lit across the countryside to welcome any friendly spirits back that popped by for a visit. In all, it’s a complicated all-encompassing start of a new year.

Today we know this as Halloween. It is just a shadow of what we once had as a culture, but the tidbits are all there. We still disguise ourselves, and we still have a large communal party, even if it is more kid oriented than before. In England, they probably have bonfires for Guy Fawkes’ Day (5 November) as a remnant of Samhain.

In America there was a more interesting spectacle in Texas, where the Scotch-Irish Celts met the Aztecs. The latter had celebrated what we know as “Dia de los Muertos” (Day of the Dead) for thousands of years. This is a day to have a small picnic by the graves of your ancestors, and perhaps have a small party with masks and images of skulls. Certainly, this festival must have been recognized immediately by Celts.

What is most striking about celebrations by “primitive” people like Celts and Aztecs is the duality inherent in major celebrations. We honor death, but we mock it. We celebrate those who went before, but we enjoy the ones who are still with us more. Beyond even the duality of life and death, however, is the universality of the practices. Everyone bows to the cycles, everyone has reason to watch out if they were not completely decent.

It may not be any great mystery that Celts and Aztecs had similar celebrations. I like to think that perhaps the wandering Celts either met or became what would later be Aztecs somewhere on the steppes of central Asia, long before we can remember. But it isn’t necessary at all. The simple fact is that there are tremendous similarities among all members of our species that operate at a very gut level. The more “primitive” we are, the more we stay close to these instincts. The differences we see are cultural, entirely made up of learned behavior.

On this day of shadows, I like to think about the duality inherent in peoples who have wandered the earth. There is a light inside all of us, but there are shadows of distant remnants of culture. Many we don’t even understand anymore. At some basic level, these shadows are all remarkably the same, as the Aztecs and the Celts showed. The shadows and the light all come back together in the end, like a circumnavigation of the globe that we all call home. One side of that is in light, one is in shadow. This time of year, it tilts us towards our shadows. They aren’t anything to be afraid of.