The events in December come with a dress code.
The Nobel events are, in varying degrees, formal occasions. We therefore ask our guests to dress accordingly. The Nobel Peace Prize Award Ceremony in Oslo is a solemn ceremony and gentlemen are expected to arrive in a dark suit and tie, ladies in a dress or equivalent. The Nobel Peace Prize Banquet is a formal affair where gentlemen are required to appear in black tie, while ladies should wear an evening gown.
Wearing your national costume is a perfectly acceptable, even encouraged, alternative at all events.