n001mary
... From late May to early July the nights are bright in St. Petersburg, with the brightest period, the White Nights, normally lasting from June 11th to July 2nd. The White Nights (Beliye Nochi) are a curious phenomenon caused by St. Petersburg's very northerly geographical location - at 59 degrees 57' North (roughly on the same latitude as Oslo, Norway, the southern tip of Greenland and Seward, Alaska). St. Petersburg is the world's most northern city with a population over 1 million, and its stands at such a high latitude that the sun does not descend below the horizon enough for the sky to grow dark. In fact night becomes curiously indistinguishable from day, so much so that the authorities never need to turn the city's streetlights on!
http://www.saint-petersburg.com/virtual-tour/whitenights/