Raising and dancing around a maypole (majstång or midsommarstång) is an activity that attracts families and many others. Greenery placed over houses and barns were supposed to bring good fortune and health to people and livestock; this old tradition of decorating with greens continues, even though most don't take it seriously. To decorate with greens was called att maja (to may) and may be the origin of the word majstång, maja coming originally from the month May. Other researchers say the term came from German merchants who raised the maypole in June because the Swedish climate made it impossible to find the necessary greens and flowers in May, and continued to call it a maypole. Today, however, it is most commonly called a "midsommarstång" (literally midsummer pole).

(from Wikipedia)