Samhain — the Celtic ancestor of today's Halloween — was both the last of the four seasonal celebrations that divided the Celtic year into quarters (see Imbolc, Beltane, and Lughnasa) and the first of their new year. The word samhain means the end of summer, but the time of year meant the beginning of winter. The Celts chose this celebrational but ominous turning point as their New Year.
Their harvest was in, their livestock were back from summer pastures, and their families were as ready as they could be for the long winter ahead. To celebrate the New Year, the Celts spent a long eerie night honoring their dead, who might be wandering around cold and lonely at this transitional time of year. They lit bonfires and prepared food for any of the dead who might come to call. Other more negative spirits might also be abroad, which added an element of fear to the occasion.
As far as today's Halloween is concerned, we have the Celts to thank for the skeletons, ghosts, goblins, and other scary or supernatural elements. We can also thank them for the the fact that we celebrate Halloween at night. Because Celtic days began at sunset, their festivals always began in the evening and lasted until well after dark.
When the Romans invaded Celtic lands, they added their own November harvest festival to Samhain. So Halloween's harvest elements — especially apples and nuts — came from the Romans. Later, when the Christians began to dominate Roman and European cultures, they added the church's celebration of dead saints and martyrs — All Hallows — to Samhain. So it was the Christians who gave us the name we now use: All Hallow's Eve modernized to Halloween. Finally, during the 1840s, the Irish fleeing their potato famine added the jack-o'-lantern to customs evolving in this country. In Ireland, children had carved rutabagas, turnips, or potatoes, but our native pumpkins made much bigger and brighter jack-o'-lanterns.
Of the Celts' four seasonal celebrations, only Samhain has persisted with some of its original power still intact. Imbolc has degenerated into a somewhat ridiculous Groundhog Day, Beltane has become an international labor day, and Lughnasa has been forgotten altogether.
But Samhain survives as our massively popular Halloween — a Celtic, Roman, Christian, Irish, and now thoroughly American celebration. It invites us, as it did the ancient Celts, to take an eerie but festive break between the ease of summer that's now behind us and the rigors of winter that loom ahead.