Any tourist visiting Norway during summer have probably seen one or more of the traditional Norwegian "bunad", which is the name of the traditional, national costumes, and asked questions about them.
Norwegians use it for various celebrations, like weddings, folk dances, and of course on the May 17., the Norwegian Constitution Day celebrations.
It's also much used on religious occasions like baptism celebrations, confirmations and Christmas.
In recent years, it has become more and more popular, even among young people, especially girls.
A typical bunad is hand made in a very traditional way, even though some companys now are having them made in China now. (Probably 2 of 3 costumes are made in China these days).