It's October. As in, Halloween! They still celebrate Christmas, so why not other holidays. 
As far as plant life 200+ years later is concerned, aside from nuclear winter and it being October again, it's 200 years of radiation storms, mutations, giant insects, warring factions, scavengers, raiders, super mutants, mini nukes, rampaging sentry bots, mad scientists, ghouls and God only knows what else. Is it really so hard to believe that the wasteland would lean just a little bit more on the scorched earth side than being a new Garden of Eden?

Did you ever see that video of Chernobyl made by the girl on the motorcycle (there's also a National Geographic that shows it as well)? Fascinating! The whole place has been taken over by lush vegetation, and I didn't see even ONE radscorpion anywhere!