Kalmar Union
The history of Sweden between 1397 and 1523
In 1388, at the request of the Swedes themselves, king Albert of Mecklenburg was driven out by Margaret I of Denmark and at a convention of the representatives of the three Scandinavian kingdoms (held at Kalmar in 1397), Margaret's great-nephew, Eric of Pomerania, was elected the common king, although the liberties of each of the three realms were expressly reserved and confirmed. The union was to be a personal, not a political union. Neither Margaret herself nor her successors observed the stipulation that in each of the three kingdoms only natives should hold land and high office, and the efforts first of Denmark (at that time by far the strongest member of the union) to impose her will on the Union's weaker kingdoms soon produced a rupture, or rather a series of semi-ruptures. The Swedes first broke away from it in 1434 under the popular leader Engelbrekt Engelbrektsson, and after his murder they elected Karl Knutsson Bonde their king under the title of Charles VIII, 1436.
In 1441 Charles VIII had to abdicate in favour of Christopher of Bavaria, who was already king of Denmark and Norway; however, upon the death of Christopher in 1448, a state of confusion ensued in the course of which Charles VIII was twice reinstated and twice expelled again. Finally, on his death in 1470, the three kingdoms were reunited under Christian II of Denmark, the prelates and higher nobility of Sweden being favourable to the union.
After the briefly successful reconquest of Sweden by Christian II and the subsequent Stockholm bloodbath in 1520, the Swedes rose in yet another rebellion which ousted the Danish forces once again in 1521, though Stockholm did not surrender until the summer of 1523. While independence was being reclaimed, the election of King Gustav of the Vasa at Strängnäs on June 6, 1523, has been seen as a formal declaration of independence, and as the de facto end of a union that had lost all long-term support in Sweden. The day Gustav Vasa was hailed as King (he was not crowned until 1528 though) would become, in 1983, the National Day of Sweden.
References: Wikipedia