Friday 14 July dawns grey and grim, more Yorkshire in March than southern France in high summer. Not a good start for the centrepiece of the millennial Bastille Day, the 'Incroyable pique-nique' - a nation-long linear banquet stretching, with a few gaps, 1,000 kilometres from Dunkirk in the North to Prats-de-Mollo, on the Spanish border, in the South. It is designed to celebrate and inaugurate the 'Meridianne Verte' - a mighty row of trees planted this year through the centre of the country along the Paris meridian in a living tribute to the millennium.

One of the areas hardest hit by the weather is the Cantal, a rural département of rounded hills and unspoilt villages at the southern end of the Auvergne. In the village of Mourjou (pop. 354), one of 337 communes along the line of the great picnic, the day begins with a flurry of phone calls. Is it going ahead or not? Two villages have cried off entirely. Who's still on? In the end Geneviève won't come to the picnic, Véronique and Michel will, and Peter (slightly grudgingly) and Richard. But we opt out of the walk which was planned to work up people's appetites. Only 30 or so brave hikers start out from the tricolour-bedecked mairie on the 10km trek to lunch.

For us, the initial omens are not good. After a 20-minute drive through the drizzly countryside, we arrive at a sodden hillside field, the original destination, to be redirected to a more sheltered spot in nearby Cassaniouze.

Rain drips from the trees and the sky is heavy as a plate of cold aligot. But inside the shelter, set with tables spread with rolls of a 600km length of red-and-white checked tablecloth, specially made for the occasion, the mood is quite different. The atmosphere is one of shared defiance and good humour, laced with the unmistakable sense of a deserved enjoyment of the rewards of shared fortitude - very much like an English picnic, in fact.

As the meal progresses, the separate groups of diners noisily coalesce. An accordionist with a weather-sculpted face looms out of the damp to play vigorous Auvergnat tunes. A Cassaniouze stalwart passes round a bottle of home-made prune wine, completing the integration process. The tablecloth is ceremonially cut up into souvenir-sized pieces. It's the longest single piece of cloth ever made by the manufacturing firm.

Back in Mourjou, after a well lubricated evening meal for 100 or so at the mairie, most of the village packs the hall to listen to a storyteller, une conteuse , who for an hour and a half holds young and old with a graphic telling of local legends and myths. In a mixture of French and Occitan (the language of Provençale), it's a remarkable performance. The biggest laugh of the evening: when she weaves into one of her tales a derogatory mention of McDonald's.

After all, this is close to Millau, where the court case against the French farmers' leader José Bové, for having 'dismantled' a new McDonald's outlet in 1998, led to a proto- pique-nique - a celebration of localness, diversity and the right to oppose everything the Great Hamburger is seen to stand for.

Whether in rainswept Cassaniouze, in Paris, or in the tiny village of Mosset where my godson Theo lives with his parents Rosemary and Miles, the pique-niqueurs play out the Revolutionary theme. Passing through 20 départements, the Meridianne on which they sit (or shelter) is itself a symbol of unity and difference, a living link from the flat fields of the Pas de Calais to the mountainous Pyrenees, by way of Paris in the middle. Local and national at the same time, the 'Incroyable pique-nique' is democratic fraternity in action.

The walkers in search of their lunch are also following a revolutionary route. For the Meridianne traces the path chosen by the revolutionary authorities in 1792 as the line along which to measure the exact distance from the North Pole to the Equator.

One ten-millionth of that length was to be the metre, the keystone of a new measurement system that (far from being the artificial imposition sometimes represented in Britain) would free the poor for ever from ' deux poids deux mesures ' - arbitrary changes to weights, measures and currency with which the ruling class had long oppressed them. The French wanted the Paris meridian, not Greenwich, to be the baseline for calculations of world time and longitude. Paris Mean Time wasn't to be, but the metric system commemorated in the Meridenne Verte remains one of the most liberating, egalitarian achievements of the Revolution.

So it may not have been a beautiful day on 14 July in a meteorological sense. Instead of the several million anticipated, only a few hundred thousand hardy picnickers defied the weather to tuck into regional gastronomic specialities and take part in concerts and other millennial events.

But then, it was never just a cheap meal, either. As anyone will tell you, philosophically it worked just fine: liberty, equality and fraternity were all out in full force at the 'Incroyable pique-nique' in the rain.