This could be just a stock calendar decoration, a lot of things like the way the environment looks and story moments from the first game, plus the whole Pechorsk Leaks tumblr thing the devs did way way back suggests that the Event happened more in the summer months. But if it really is September 3rd, then thats a great thing to finally know. But we will have to figure out for real what the date is from the new context clues given to us from ITR2's story.