"Journee du Poilu" means Poilu´s holiday, since Poilu (hairy one)used to be called the French World War I soldier. The "Journees" were important to mobilize the financial and moral support to the French fighters. Buying and wearing these medals was a gesture of patriotism and support for them.
Here an interesting site about the "Journees":
http://www.plugged-creations.com/jo.../accueil.swf
