Flogstaskriket, which translated from the arcane Swedish language,
means 'the Flogsta Scream', is a custom for students of Uppsala Universitet that dates back to time
immemorial (according to the students I talked to, it's been a custom
since at least the 19th century, possibly earlier).
At ten PM every evening, students ascend in droves to the rooftops
and rafters of their residences, and proceed to let loose a
bone-pulverizing scream. During the academic year is the best time to
participate in Flogstaskriket. The fun is particularly enhanced by
vodka (which, thanks to the government monopoly on alcohol in
Sweden, is better purchased in Åland --- a clump of rocks owned by
Finland in the Gulf of Bothnia --- than anywhere in Sweden).
Theories abound as to the origins of Flogstaskriket. Some say it began in the 20th century as a stress-relief activity during exams week, which, as my then-girlfriend kindly informed me, is very much inducive to screaming at the top of your lungs. Others say it began as a sort of memorial to a student who jumped to his death from the rooftops.
Still others shrug and say it has basically always been part of the Uppsala culture, which makes some sense, when you realize that the Swedes have done pretty well for a country that experiences months of depression-inducing premature sunsets at the mercy of the unholy latitude at which the country lies. Your humble author and amateur ethnographer would also like to remind you, that while the original inhabitants of Scandinavia were particularly fond of screaming, and despite the fact that Sweden has such luxuries as 24-hour ATMs, mass transit that actually works pretty damn well and effective universal health care, the Vikings spirit has never quite left the average Swede.