The original Sedan Day was the date of victory by the Prussians over the army of the French Emperor Napolean III. As noted in the diary by William, "At 10 A.M. the glorious official news is received that Emperor Napoleon with the whole army under MacMahon, who is wounded, capitulated and are now prisoners in the hands of the King of Prussia, at the Fortress Sedan, near the Belgian frontier. We are all mad with joy and excitement" (Diary, 1870-09-03) In subsequent years, the anniversary of the victory was celebrated in Germany and by German-Americans.
The Franco-Prussian War, which ended with the surrender of the French at Sedan, was sparked by the French determination to prevent the son of King William from being a candidate for the vacant throne of Spain. The French did not want to face German power on two frontiers, the Pyrenees and the Rhine.(1) Six weeks after France foolishly declared war on Prussia in July of 1870 (tricked by Bismarck into thinking that Prussian King William had insulted the French Emperor Napoleon III), the French army and its Emperor were surrounded in the fortress town of Sedan, near the Belgian border, and were forced to surrender.(2) This Prussian victory concluded the Franco-Prussian War. It had a political counterpart, in that Bismarck used the war to complete the unification of Germany, creating the German Empire and elevating King William to the status of emperor.
Shortly before the outbreak of war, King William visited the vacation resort of Ems; the French ambassador approached him and attempted to obtain a commitment that the king's son would not be a candidate for the Spanish throne. The king was offended by the French demand and refused to discus the matter. Following this meeting, the king sent a telegram to Bismarck in Berlin describing the encounter. Bismarck edited the telegram to make it appear insulting to the French and released it to the press. The French public reacted in a chauvinistic frenzy. The government of Napoleon III declared war on Prussia six days later.(1) This was what Bismarck had intended: a war that would unify the German states behind King William, leading to the creation of the new German Empire with William as Emperor.
When traveling in Germany, William Steinway attended many Sedan anniversary celebrations with parades and illuminations. (Diary, 1885-09-02, 1890-09-01, 1892-09-02) The German community in New York also celebrated the victory at Sedan. William often noted these celebrations in his diary. In January of 1896 he attended a concert at Carnegie Hall honoring the German victory at Sedan, which included an original composition by Heinrich Zoellner, called "Bei Sedan" [By Sedan]. (Diary, 1896-01-17) William closely followed reports of events in Europe, and particularly the news from Germany. He frequently mentioned these events in his diary, often without comment.