In the Baltic states, a positively pagan vibe sets in at the summer solstice. Cities empty. Cars, bikes, horses and yellow public buses are decorated with flowers and oak leaves. Urban types take to the countryside to drink local beer, eat special "midsummer's cheese", build campfires and sing and dance all night long. My Russian friend Vanya, who grew up in the Baltic state of Latvia, gets a little misty-eyed when he remembers the special songs sung only on the night of June 23-24, the festival known there as Ligua. But he won't sing them himself except on the special day.

That's natural enough in nearly Arctic countries where the sun really stops setting at midsummer and the days start going on all night in a mysterious and mischievous way. After long winters whose days are very short and not very light, no wonder people want to celebrate a festival of light.

But you'll find people in the wild places of the south doing very much the same thing during the solstice. In the remotest part of southwestern France, the countryside behind Carcassonne, for instance, where Vanya and I and both our families are on holiday this week, they build giant bonfires on Midsummer's Eve. After downing large quantities of pink wine, they jump right through the flames to cleanse away the past and start a new life. (Naturally, with drought looming and the whole area bleached dry as a tinderbox, the pompiers are scared stiff of the "famous night that is neither long or short". As far as they're concerned, the night is both long and dangerous. But that's the power of ancient ritual for you).

Whether it's north or south, I would probably have assumed that midnight bonfires and oak-leaf wreaths were the kind of thing that would be of more interest to people in the parts of Europe to which iPods  haven't yet spread. And yet, the night before going on holiday, June 21, I was called out for an evening picnic on Primrose Hill to celebrate Midsummer (there seems to be some confusion about the night) - and saw very similar things happening even in that sleek London district, so packed with urban shopping opportunities and movie and music stars that it can outshine Aurora Borealis.

If you had climbed out of the city traffic to the top of the hill in the scented dusk, as the sun went down on that hot evening, and watched the fire of the sunset reflected in every glass skyscraper window in the distant City, you'd have seen the skyline of central London go red, then a giant full moon - also blood-red - rise between the BT tower and the London Eye.

It started by looking just like a Primrose Hill gathering. Everyone picnicking on the hill between 9pm and midnight was just about plus or minus 30. Everyone was slim and beautiful and expensively casual in jewelled sandals - a host of Jude Law and Sienna Miller clones.

On the right (and perhaps Right) were pretty bohemo-sloanes drinking champagne and eating strawberries and discussing their book deals, or making up to the rather older BBC producers who pay a pittance for a play by a bright young thing. In the middle was a dancer, swooping flags. And on the left (and maybe Left)  was a a collective picnic of 20 or more people, with drums and guitars and rhythm, wearing a lot of white muslin and very clean tiered peasant skirts and bared midriffs and white rasta dreadlocks. Over here there was a whiff of cannabis and a strong suggestion of wholesome organic breakfasts.

But little by little everyone got drawn into the same magic.

"Look at the extreme moonrise!" everyone on my vague picnic kept calling, with the same very unvague sense of wonder; even the most flamboyant guest, a photographer called Guy, who for some reason was wearing an 18th-century lilac frock coat and who'd seemed at first as if he might be too cool to notice. And wherever we looked on the hilltop, people were stopping and taking pictures of the sky on their mobiles. Behind was the vague beat of drums, and the flames of a tinkerish fire half way up the hill, on which I hoped that someone might be roasting a hedgehog. And it was strangely reassuring to see that everyone in the city, even the super-sophisticates of Primrose Hill in their jewelled sandals, would still want to come together in this very traditional celebration - an epiphany, a gesture of awe at the profoundness of nature.

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