Technically speaking the so-called "White Nights" are not unique to St Petersburg, but only here have the northern lights received such a poetic acclaim. It is beautiful walking along rivers and canals when night is as bright as early evening? It is the world's only metropolis where such a phenomenon takes place every summer.
From late May to early July nights are bright in St Petersburg, but the real White Nights normally last from June 11 to July 2. The nature of the White Nights (Beliye Nochi) can be explained by the geographical location of St. Petersburg. It is the world's most northern city with a population over 1 million. St Petersburg is located at 59 degrees 57' North (roughly on the same latitude as Oslo, Norway, the southern tip of Greenland and Seward, Alaska). Due to such a high latitude the sun does not go under the horizon deep enough for the sky to get dark. The dusk meets the dawn and it is so bright that in summer they do not turn street lighting on. If the weather is cloudy you might not see any White Night, though on a clear day you can get a feel for a White Night even in mid-July.
WHITE NIGHT IN ST. PETERSBURG, RUSSIA