This article is from the Nordic countries FAQ, by Antti Lahelma and Johan Olofsson, with numerous contributions by others.
Val Davies <val@altinkum.demon.co.uk> wrote:
I recently came across a reference to an occasion called "Valborg" and
on looking it up in the dictionary find that it apparently translates
into English as "Walpurgis Night". I confess to being none the wiser.
:-(
[ Henrik Ernoe: ]
Valborg is the Scandinavian name for the Catholic Saint Walpurgis.
Walpurgis is believed to be the patron of witches (this is of course
not certified by the Church). Her day is the 1st of May. Walpurgis
night is the night before May 1st. On which nature was suppossed to be
potent. So if a girl wanted to get pregnant the following year she
would go and bathe in a holy well or creek on that night. There was
also a number of magical rituals supposed to make livestick fertile
that were carried out on Valborgs eve.
[ Antti Lahelma: ]
It's the 1st of May. A important holiday in these parts; you wear a
white student cap (supposing you ever graduated), a silly nose
(optional), drink a whole lot of alcohol and walk aimlessly in the
crowd downtown. In Helsinki, one of the main events is the crowning of
a statue of a mermaid (Havis Amanda, a symbol of the city) with said
white cap. I presume it's old pagan festival to welcome the spring;
the Christian excuse for celebrating it has to do with a certain St.
Valborg, a German 9th (?) century abbess who probably did something
pious that has nothing to do with Valborg (Vappu in Finnish) as we
know it.
[ Alo Merilo: ]
In Estonia the Walpurgis Night (in Estonian "Volbriöö") is basically
when all self-respecting present or past university students who
belong to either a fraternity /sorority ("korporatsioon") or a student
society, have probably the biggest party of the year. The tradition
probably has its roots in Germany.
[ Johan Olofsson: ]
The festival has its roots in on of the pagan rites to honour the
return of Spring. In Sweden the important part is the Eve, the last
day in April, when people make big bonfires and greet the Spring with
a lot of singing.
 
Continue to: