The draft is long gone but that does not mean that military service (defense of the nation) has been disconnected from citizenship. The JDC is part of an overall effort, a "parcours de citoyenneté," on the part of the state and the military to "informer les jeunes Français sur leurs droits et devoirs en tant que citoyens pour les aider à mieux comprendre le fonctionnement des institutions de leur pays. (Inform young Frenchmen and women of their rights and duties as citizens to help them better understand how their country's institution function.)
The JDC is actually the last step in this process which starts in high school. Sometime during the third and first year, the professors are required to include as part of the program of civic, legal and social education, discussions about national defense, state security and what dangers and challenges exist in the world today. The second step is the census. At the age of 16 all young Frenchmen and women are required to present themselves and be counted at their local mayor's office. This is a pre-requisite for being able to pass the national exams (CAP, BAC, driver's license).
The final step is a one day session for all young men and women who have reached their majority (18 years). The objective of this day is to point out a very important and essential fact:
Les pouvoirs publics et les forces armées agissent chaque jour pour que la liberté puisse exister, sur notre territoire, mais également en Europe et sur d'autres continents.
La JDC est une journée qui permet de rappeler à chacun que cette liberté a un prix. C'est aussi une occasion unique de contact direct avec la communauté militaire, et de découverte des multiples métiers et spécialités, civiles et militaires qu'offre aujourd'hui aux jeunes, la Défense.
The forces of order and the armed forces act every day so that liberty can exist, on our territory, but also in Europe and on other continents.
The JDC is a day to remind everyone that liberty has a price. It is also a unique occasion for direct contact with the military community and to discover the numerous profession and specialties (civilian and military) available today for young people in the armed forces.
You can read more about the actual program here. They provide breakfast and lunch, basic first aid training and even a visit to a military installation in addition to talks about the rights and duties of citizenship and national defense matters. Naturally a bit of recruiting is slipped in there as well.
What do I (US citizen, long-term resident of the French Republic and potential citizen) think of all this? Well, I rather wish that my country had done something similar for me in my formative years. I do not recall (perhaps this has changed) anything quite so comprehensive and clear concerning citizenship being provided as part of my education. And though I did come from a part of the U.S. that has quite a few military installations, the first recruiter or veterans I ever encountered, I met in my second or third year of university. As odd as this may sound the only military personnel I've ever known well have been French: my father-in law (career French Army officer) and the elder Frenchling's god-father who is an officer in the French Navy (lots of uniforms at my wedding). Even with that experience, I would not pretend to know or understand very well what they did or do.
I wish I could participate in one of these National Defense and Citizenship Days. As an observer certainly, who is genuinely curious and interested in what they have to say, but also as a potential citizen who has read the Charte carefully and the part that says: "Tout citoyen concourt à la défense et à la cohésion de la nation" (Every citizen contributes to the defense and the cohesion of the nation).