December 21 is the winter solstice, the shortest day of the year. And it is why we need the twinkling lights of Christmas.

The winter solstice brings depressing darkness to the northern hemisphere, and the farther north you go, the shorter the days become. Boston, at 42 degrees latitude, is closer to Rome and Madrid, than to London or Scandinavia where there may be only 2 to 4 hours of sun a day.

For early cultures it was a mystical time, for the earth was dark and barren. Plants and trees lost their leaves. The goddess of the harvest slept, or maybe even died. Very scary.

But some insolent plants held on to their green leaves, defying the evil spirits of the dark and cold. Druids, Greeks and especially Romans worshiped these brave evergreens. They cut them, decorated their houses with them, and also drank a lot of wine, presumably for warmth and reassurance.

And so, they became the brave evergreens of Christmas, holly and ivy and mistletoe.

Poisonous all, they were used medicinally by early priests and shamans. Mistletoe, a parasitic weed of trees, was sacred to the Druids. White-frocked priests plucked it with a golden sickle to cure illness, infertility and pacify one’s enemies. To kiss beneath it ended grievances, which by some mysterious convolution, became today’s romantic custom.

Pagan tribes in England and Germany kept holly to ward off evil spirits, bad weather, and to protect a maiden’s virginity. (How does sex always end up in these folk tales?) These myths have evolved into our winter customs, continuing their reassurance that indeed spring would come again.

Before electricity those nights were long and dark. Very dark. (Try the inside of a closet.) Illuminating the darkness was reassuring and protective against irrational fears. It still is.

So at this winter solstice, the joyful holiday lights are most welcome – to reassure and comfort us – and make our towns a twinkling wonderland. And I do as much shopping as possible at our local stores. Their holiday lights cheer, for me, the depressing darkness of these shortest days of the year.