The days between March 20, 1815, when Napoleon reached the Tuileries, after his escape from Elba, and June 28, the date of the second resioration of Louis XVIII. These hundred days were noted for five things:
The additional Act to the constitutions of the empire, April 22; The Coalition; The Champ de Mai, June 1; The battle of Waterloo, June 18; The second abdication of Napoleon in favour of his son, June 22.
He left Elba February 26; landed at Cannes March 1, and at the Tuileries March 20. He signed his abdication June 22, and abdicated June 28.
The address of the Count de Chambord, the prefect, begins thus: “A hundred days, sire, have elapsed since the fatal moment when your Majesty was forced to quit your capital in the midst of tears.” This is the origin of the phrase.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894