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. 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. The Nobel Peace Prize concert is less formal, but we do appreciate our guests dressing up for the occasion.
Wearing your national costume is a perfectly acceptable, even encouraged, alternative at all events.