The sky is behaving strangely. It is swirling behind the dome of St Isaac's Cathedral – its misty, gossamer brightness condemning the grandest of St Petersburg's vast churches to silhouette. And there are dabs of blue amid the clouds – shards of turquoise-azure which hint at a time somewhere near 3pm in autumn. I check my watch.
It is sticking to its story – that the hour is well past 11pm, and yet the day is still with us.
Russia's former imperial capital is surely the most splendid urban setting for a date with the midnight sun. True, Helsinki and Oslo also enjoy the "White Nights" that grace the upper portions of Europe in high summer. But nowhere does this carnival of light seem quite so dramatic as amid the architectural flamboyance of a metro-polis that was dreamt up by Peter the Great in 1703 – all epic palaces and open spaces on the banks of the River Neva.
The White Nights period is an intoxicating experience in St Petersburg – and a lengthy one too. Between now and 17 July, the sun will not set before 11pm over a place that dots the map at 59°56N – and it will take until 14 July before it is so tardy as to be back above the horizon later than 5am. Between these hours, the city is bathed in a persistent pale lustre that gradually dims to gloom, but never wholly surrenders to black.
The result is a season that is embraced by a city that endures six months of winter – 50 or so days when St Petersburg forgets about thick coats, sturdy hats and traditional Russian reserve, and instead acts with the giddiness of a gaggle of pupils on the last day of term.
Indeed, "Scarlet Sails" (Alye Parusa) – the centrepoint of the festivities that illuminate the city between late May and late July – is just that. Scheduled for 11pm next Saturday, this riverside party is a celebration of the end of the school year. Tied to the 1922 children's book of the same title – written by the author Alexander Grin, whose fairytales injected drops of the fantastical into the austerity of post-revolutionary Russia – it will see a tall ship, masts clad in fiery red, glide down the Neva as a metropolis rejoices on the south bank.
Along English Embankment (Angliyskaya Naberezhnaya), bands will strum, orchestral ensembles will perform and fireworks will explode as boat races and "pirate battles" play out on the water. In a way, this is a New Year's Eve extravaganza staged at the heart of the calendar, a sign of the happy topsy-turviness that engulfs St Petersburg in summer.
There are other, less cacophonous notes to the season. Since 1993, the city's artistic icon, the Mariinsky Theatre, has proffered the Stars Of The White Nights festival – a flurry of elevated culture. The 2013 run of shows continue until 28 July with ballet staples such as Swan Lake and opera cornerstones like Tosca and Aida – but will come with added sparkle this year because the striking new glass-and-limestone venue, Mariinsky II, which opened last month, will also be used.
The merry atmosphere is palpable whether you are drinking in one of a crop of stylish watering holes – such as the DJs-and-cocktails enclave of Clean Plates Society on Gorokhovaya Ulitsa – or dining at the Astoria Café in the sacred confines of the Hotel Astoria – the accommodation giant that has just marked its 100th anniversary.
Strolling to the river from the latter just as 1am dawns, I realise that the joy even envelopes the city's municipal rituals. From May to November, when the flow is free of ice, the Neva's bridges are raised during the small hours to let boats pass to sea. A crowd has gathered, awaiting the lifting of the Blagoveshchensky and Dvortsovy bridges, the two most central structures, which spread arms in unison at 1.25am.
As the moment arrives, a cry goes up, and the music from the food stalls and ad-hoc bars on the edge swells. Here, it seems, is the answer to the question of what happens after dark when, strictly, there is no darkness.
British Airways (0844 493 0787; ba.com) flies to St Petersburg from Heathrow.
Doubles at the Hotel Astoria (007 812 494 5757; thehotelastoria.com) start at £161, room only. Four nights with breakfast, transfers and BA flights from Heathrow, cost from £850pp through Exeter International (020-8956 2756; exeterinternational.co.uk).
Mariinsky Theatre, Teatral'naya Ploshchad 1 (007 812 326 4141; mariinsky.ru).
Clean Plates Society, Gorokhovaya Ulitsa 13 (007 812 934 9764; cleanplates.ru). The locally based Saint-Petersburg.com office offers more tourist information.
British passport-holders require a visa; £77.60 (0905 889 0149; ru.vfsglobal.co.uk).
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