According to my point of view, 17th of May in Norway is the best day in Norway! Norwegians celebrate their independence, but to be more specific, adoption of the Constitution of Norway (16 May 1814), according to which Norway is a constitutional monarchy.
People begin to drink champagne and eat strawberry from 9.00 a.m. Then go to the city-center to look at parade and salute the Royal family. Honestly, I like the Norwegian Royal family. They are so close to the people that they want to show: they are also people, not gods. (For example, like in the UK ;) )
So the main attributes of 17th of May in Norway are champagne, fun, friends, family, parade, and, of course, traditional costumes. It is so great that people do not forget their roots and proudly show their belonging to those place where they were born.
The most important is not to drink too much, otherwise... ;)