31 de Julho de 1919.

German national assembly adopts the Weimar constitution (to enter into force August 14). The Weimar constitution (German: Weimarer Verfassung) was the document that governed the short-lived Weimar Republic (1919-1933) of Germany. Formally it was the Constitution of the German State (Die Verfassung des Deutschen Reiches) but it shared this title with the imperial constitution that preceded it.

Following the end of WWI, a German National Assembly gathered in the town of Weimar, in the state of Thuringia, in January of 1919 to write a constitution for the Reich. The fundamentals of the constitution were known: It was to be a Democratic, Federal Republic, with a President and Parliament to govern it. Supreme power was to be delegated by The People.

Disagreements arose between the delegates over such things as the new national flag, religious education for the youth, and the rights of the regional states that were to make up the Reich. All of these disagreements were resolved by August of 1919.

The document was divided into 2 parts. Each part was divided into sections (7 sections in Part I, 5 sections in Part II). In all, there were almost 200 articles in the Constitution. In short, they declare Germany a republic which derives its consent from the people in accordance with international law and natural law.

The main tenets of the Constitution are as follows:

The President was to be elected for terms of seven years. There were no limits placed on terms. In addition, he was to have emergency powers, including the ability to dissolve the Parliament, the ability to unilaterally issue emergency legislation, and the ability to deploy the armed forces to restore order within the Reich.
The Parliament was to be elected for terms of four years. Additionally, it was to function as a Proportional Representation System.
The voting age was set at 20, with universal suffrage.
The Constitution guaranteed individual rights such as the freedom of speech and assembly to each citizen. These "Fundamental Rights And Duties of Germans" were based on the provisions of the earlier constitution of 1848.

Sixty-seven delegates abstained from voting to adopt the Constitution.

The 1919 constitution had a number of fundamental weaknesses, which made the establishment of a dictatorship all too easy, such as Article 48. Whether a different constitution could have prevented the Third Reich is debatable though. Upon Hitler’s rise to the Chancellery in 1933, and his subsequent seizure of the Presidency, the Constitution was abused. After the dissolution of the Parliament, the suspension of civil rights through the Reichstag Fire Decree, the Constitution was only a dead form but it was never formally revoked.

Wikipedia

More of the Weimar Constitution here.