VisitNorway have a good explanation of Norway’s national day here, if it is something you’d like to read more about. And the people behind the 17th of May Festival in Seattle (Ballard), Washington, have made a more in-dept explanation about the history of it, which you can find here.
In Norway it’s common to congratulate people with the day, as it is one of the most special days in the year. So congratulation to Norway, and the people of Norway.
(Here’s the post from last year, and this picture was also taken on the 17th of May 2013 in Stavanger.)