The day is celebrated in honor of the election of Gustav Vasa as King of Sweden in 1523 and of the adoption of the constitutions of 1809 (which was replaced in 1974)
Prior to 1983, the day was celebrated only as the Swedish Flag Day (Swedish: Svenska flaggans dag). It was officially named the Swedish National Day by the parliament of Sweden in 1983, and became a public holiday in 2005.
I have to ask uncle Google about why you celebrate that day. Actually it's quite interesting. Tonight after work I will do some more reading about that
