History of Germany during World War I
In November 1918, with internal revolution, a stalemated war, Bulgaria and the Ottoman Empire suing for peace, Austria-Hungary falling apart from multiple ethnic tensions, and pressure from the German high command, the Kaiser and all German ruling princes abdicated. On 9 November 1918, the Social Democrat Philipp Scheidemann proclaimed a Republic, in cooperation with the business and middle classes, not the revolting workers. The new government led by the German Social Democrats called for and received an armistice on 11 November 1918; in practice it was a surrender, and the Allies kept up the food blockade to guarantee an upper hand. The war was over; the history books closed on the German Empire. It was succeeded by the democratic, yet flawed, Weimar Republic.
Day of the founding of the German Empire
The North German Confederation had already officially adopted the name "German Reich" in its Constitution by 1 January 1871, so constitutionally speaking, 18 January was not the day of the founding. 
Celebrations were held annually on 18 January. At centralised and local events, patriotic speeches were held and songs such as "Heil dir im Siegerkranz", the unofficial anthem of the empire were sung. Celebrations also took place publicly during the Weimar Republic (1919-1933) and with the participation of high dignitaries. Nazi Germany only celebrated the founding of the German Empire in 1871.