Comfort and joy that will make you weep

'Tis the season to be merry, but for many, the annual carol fest brings tears to the eyes. Is it because we're reminded of our childhood? Or of something more primordial?

It wasn't until I had a child that I realised what carols can do to you. My primary-school headmaster was wonderfully mad. He had a forehead as round as a pudding, he ranted in assembly, and locked blonde girls in cupboards. My friend Dinah said he was paranoid. I didn't care, I was dark, and he was giving me (though he didn't know it) a gift for life. He drilled us in old spirituals, sea-shanties and carols. Never mind what words meant, they belonged to magic melodies, harmonies mysteriously linked to stained-glass-window moments in a shadowy story.

It wasn't until I had a child that I realised what carols can do to you. My primary-school headmaster was wonderfully mad. He had a forehead as round as a pudding, he ranted in assembly, and locked blonde girls in cupboards. My friend Dinah said he was paranoid. I didn't care, I was dark, and he was giving me (though he didn't know it) a gift for life. He drilled us in old spirituals, sea-shanties and carols. Never mind what words meant, they belonged to magic melodies, harmonies mysteriously linked to stained-glass-window moments in a shadowy story.

"Masters in this hall, hear ye news today?" (Who were they?) "Down in yon forest there stands a hall. The bells of Paradise I heard them ring. It's covered all over with purple and pall." (Sir Galahad?) "Lord Jesus hath a garden full of divers flowers." (What were divers doing in a garden?)

My blonde friends became allergic to singing, I got hooked on it for life.

Wherever I've lived since, I've sung. Verdi in an Istanbul nightclub, "Early One Morning" on Belgian radio, plainsong in the Paris church of St-Eustache. But it's in English, at Christmas, where the singing bug really bites. Tonight, I'm missing annual carols with my most loved choir, in Cambridge.

It was while I was singing in that choir, at the time my daughter was born, that I first realised carols have this awful power to make you cry. On her first birthday, we moved to an area of the city which used to be an ancient orchard. The house was terrible, the garden gorgeous. One night I came home after a carol concert and looked out back at an ancient holly tree. Frost on the ground, stars in black sky, kitchen light on holly leaves, and the "St Day Carol" running through my head. "The first tree in the greenwood, it was the holly."

Tears flooded my cheeks: words, tune and image seemed to reach right back into everything I was, but forget about being most of the time.

"How strange, the potency of cheap music," muses someone in Noël Coward. Carols always were cheap - in the sense of popular. Like the blues, they came from below, from the country. Caraulae (from Greek choros), first meant "dances", which people yearned to do at religious festivals. But seventh-century priests said they were the devil's invention.

In the 15th century, humanism came in from Italy and France. Carols are all about our European connections, a whole melodic EC of tunes and lyrics. "Angels from the realms of glory", a French carol, got English words in the 19th century. All over 15th-century Europe, people sang in their own language or mixed it with Latin. Hence the mix of "In Dulce Jubilo", a German 14th-century carol put into English a century later.

These carols were a people's revolution ("God today hath poor folk raised, and cast adown the proud"), claiming the right to sensuousness, to lyrics, melody and harmonies you create yourself in songs mixing worship with eating ("the goose is getting fat"), drinking, decoration. They appeared between 1500 and 1647, when the Puritans abolished Christmas. They were England being (as it started) Catholic: the vivacious, laughing, feeling end of worship, rural, regional ("The Sussex Carol", "Carol of the Nuns of Chester"), and dancingly democratic - at a time when democracy in Britain was just a century-old twinkle in Wat Tyler's eye.

Then they went underground. They nearly sank without trace in the 18th century, but went on being sung all over, in different counties. Carol broadsheets, with crude woodcuts, were sold at village shops. The first carol collection was 1822; its editor said carols were "a thing of the past". Yet the folk-song movement (swelling till 1898, when the Folksong Society began) started focusing on carols. In 1865, another collector was teaching carols to millgirls in the West Riding. Top broadsheet favourite was "The Cherry Tree Carol", where Joseph is grouchy to Mary. She asks for cherries; Joseph, "with answer most unkind", says: "Let him give thee cherries, Mary, that did thy body bind." After one line, the Yorkshire lasses bellowed "Nay! We know a one a great deal better nor yond."

So Yorkshire didn't need its carols reviving - but elsewhere the Victorian revival, while it did bring the schmaltz into carols, also saved them, making carols the weird mix they are today of melody, exoticism, appetite, history, harmony, convention and a tuckerbag of sentiment, where nursery rhyme, folksong, JS Bach and Sir Cliff Richard meet.

Still, why should they make me cry? Because my daughter was so young, and the holly tree and carol so old. Carols link you with children and the future, but also with the hoariest images of the most ancient England imaginable. "Carols make you think of your own childhood," said one friend; they also make you think of childhood itself and what it stands for. Beginnings: everything glittering ahead. The starry carol-night of "bleak midwinter" is endless promise. Maybe that's the essence of truly popular music: everyone's promise, made melody. Carols suck at every moment of the Christian drama, and hook it into childhood: all that open-eyed gazing at light from the dark.

This is the big carol theme: following a dream and finding it in your own world. That's why home vegetation like holly trees, wildly remote from the land of the story, is important. On a poetry tour in Israel, we got taken from Nazareth to Bethlehem. "A long way," I said during the car ride through a bleached, stony landscape. "Yes," said my friend. "Especially if you were pregnant on a donkey."

In their fantasies of Israel, carols tie you in to the ancientness of your northern land. What was it like when missionaries exported to new, hot lands carols speaking of Jerusalem through mistletoe and figgy pudding? But though the vegetable life is different, the carol symbolism of hope is universal. The bud on the bare bough is a sign of spring; the winter festival forecasts light coming to all that is dark in your soul and life. Carols are about personal "comfort and joy", about getting helped on your journey like King Wenceslas's page ("Sire, the night grows darker now, and the wind blows stronger").

Yet many carols, especially old ones, are also about accepting suffering. Some are in major keys ("The First Nowell") but many have the sad unfinished business of the minor. The shadow hanging over the Nativity is the Pietà . Same two people, same attitude (son on mother's lap), and the child's death between. Carols snap together, like poppers, Christ's swaddling and shroud. The first verse of the "St Day Carol" is "The holly bears a berry as white as the milk, Mary bore Jesus who was wrapped up in silk". The second runs: "The holly bears a berry as green as the grass, Mary bore Jesus who died on the cross." Carols remind you that on this earth promises don't get kept. A baby is the image of new life to be lived, but every baby, not just Christ, will suffer and die. All homeless kids on the streets tonight had that baby glow of promise about them once.

Why cry at carols? You're crying at every dream of following a star. The sentiment is stamped deep into what we feel about children. So kids reply with the rudest versions they can, from three-year-old giggles at shepherds washing socks to eight-year-old sophistication ("We three kings from orient are. Selling knickers threepence a pa'r, How fantastic, No elastic, Very unsafe to w'ar"), and a "Jingle Bells" doing the rounds among 12-year-olds this Christmas which is far too filthy to repeat in print.

Rude - but well in the spirit of ancient caraulae forbidden by the seventh-century Church. For despite the minor keys, carols are also about laughing, about democratic snooks at authority.

How my bossy headmaster would have hated me saying that.