Wikipedia
Day of Sedan was a semi-official memorial holiday in the German Empire celebrated on the second of September to commemorate the victory in the at Battle of Sedan (1870). After the outbreak of the Franco-Prussian War a few weeks later, French emperor Napoleon III and his army were taken prisoner in the fortress of Sedan by Prussian troops, a major step to eventual victory.
In 1871, the now united Germans could hardly agree on a common German holiday. While the German Emperor and Empire was proclaimed on 18 January 1871, the Prussians themselves held the first coronation of a Prussian king on the same day in 1701 in higher esteem. The signing of the final peace Treaty of Frankfurt, several months later on 10 May 1871, was also not unequivocally welcomed. The southern states of Bavaria, the Grand Duchy of Baden and Kingdom of Württemberg preferred to celebrate the victories in battles to which their troops had contributed significantly, such as the Battle of Wörth, which had occurred already on 6 August 1870.
While never proclaimed officially, participation and official support for Sedantag celebrations varied over time, and working class leaders never really accepted it, Sedantag became a de facto national holiday, last celebrated in 1918.
After the Treaty of Versailles had been signed in mid 1919, on 27 August 1919 the Ministry of the Interior of the Weimar Republic declared that no further Sedantag celebrations should take place.