November the 11th is in a number of European countries (as well as in the US and the Commonwealth countries) the national holiday, or even if not a holiday it is the day when different kinds of celebrations and commemorations are taking place. In the UK and the Commonwealth it is called the Remembrance Day, in France and Belgium it is Armistice Day, Independence Day in Poland and Veterans Day in the US. In each of those countries the celebrations or commemorations differ, but all of them have the same origin.
The 11th November is a symbolic date of the end of the World War I. On this day on in 1918 in an early morning an armistice (a formal agreement to stop fights) was signed between the allied forces and Germany. The agreement was supposed to enter in force on the same day at 11.00.
On the picture, Ferdinand Foch, a French marshal, the signatory of the armistice on behalf of the allied forces. A statue in Paris.
In most countries (with exception of Poland) this day is associated with commemorations of soldiers who fell on battlefields of the World War I as well as other wars or fronts. Because of the first time usage of machine guns at that big scale, the number of dead and wounded among soldiers in the WWI was extremely high and unprecedented in the human history.
In Poland, as in other countries there is a wreath lying ceremony at the tomb of the unknown soldier, but the main celebrations are about independence that was regained after more than a century of foreign dominance.