On the night of April 30, known as "Walpurgis Eve", the devil is abroad and all evil things of the earth and air and water hold revel - or so Bram Stoker wrote in one of his books about Dracula.
In Finland today this legend is a reality - at least for those who consider binge drinking evil.
For everyone else, the festival on last day of April and the first day of May (called "Vappu" here) is the biggest and best party of the year, when most upstanding Finns do their best to drink themselves horizontal.
April 30 was first celebrated by Vikings in Pagan ceremonies, by lighting huge bonfires to scare away the evil spirits of winter and hasten the coming of spring.
Since then, beginning with the Catholic Church in the seventh century, almost every group in Finland from students to Satanists and workers to winemakers have adopted the date for their own ends.
Happily, aside from the occasional protest march, the festival is today dominated by past and present students.
As it was my first Vappu, I was excited to see what all the fuss was about.
In the last few weeks of April, it had dominated conversation. Long-range weather forecasts were checked and rechecked, alcohol was stockpiled, picnics and parties were planned.
I was not disappointed.
In Finland, it is traditional for students to receive a cap when they graduate from school. With the black band around the edge and a little badge in the middle, the cap looks a lot like a captain's hat.
The festivities began at 6pm on April 30 - as they do every year - when one of these caps was placed on Helsinki's favourite work of art, a statue of a naked woman known as Havis Amanda.
This year, in the late afternoon sun, 50,000 gathered to watch Havis put something on. As 6pm approached, the crowd began to cheer, and wave their caps above their heads.
Through the forests of arms and caps we could just make out a group of students - each suspended by a rope from a crane hovering in the air.
From the moment one of them put a cap on her head, the cheering from the crowd was augmented by the popping of many thousands of champagne bottle corks.
Everyone with a hat now donned it: The party had begun.
For the next 30 hours Helsinki was awash in a sea of people and flooded in booze.
Even though I knew what the white caps symbolised, to me it still seemed like the city had suddenly been invaded by sailors on short leave.
Not long after the capping ceremony, we retreated back to our house for a party.
The bars were full and we wanted to avoid the packs of underage drinkers roaming the streets - April 30 is usually the first time young Finns ever get drunk.
Although I wanted to go out and witness the festivities first hand, at about midnight, with the party still going on around me, bed beckoned and my lens on Vappu prematurely faded out.
The next morning our alarm woke us at 7.45am. After remembering where we were, Sanna and I stumbled out of bed, knowing we had a big day ahead. April 30 is a big night for the kids, but May 1 is the main event.
We divided our time between the Swedish-speaking hangout Kaisaniemi Park and Kaivopuisto Park, where crowds as big as 100,000 gather.
Every Finn who I had spoken to about Vappu had told me that the key to enjoying May 1 is to start drinking again as soon as possible. The hangover is part of the fun.
And perhaps because I managed to get a decent sleep I actually felt pretty good: The sun was shining, the sparkling wine was flowing and the birds and Finn-Swedes were singing. Aside from temporary lows and waves of nausea it was great.
From early in the morning the city was full of drunken sailors, but I did not see any hint of unruly or antisocial behaviour.
The atmosphere was instead entirely good-natured and social, right up until the point Sanna and I decided to head for home at 9pm.
It wasn't until the next day that I discovered the evil side of Vappu.
I could smell it, even before I opened my eyes. Still festering in sunlight and central heating was the debris from our party two days earlier.
After cleaning up we met some friends in a park near our house.
It too resembled more a rubbish tip than a park, with an army of scavenging seagulls replacing the drunken sailors of the day before.
It would take an army of 50 cleaners two days to clean Kaivopuisto Park alone.
I didn't really feel like staying out anyway: The park seemed cold and hollow, and somehow dark, even though the sun was shining again.
It was almost as if it was a reflection of the way I was feeling.
Vappu was over. I couldn't wait for next year.
- Matt Kennedy-Good
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Pictured above: With 50,000 people crammed in to see the capping ceremony, seating was at a premium. Photo / Matt Kennedy-Good