Say when your Pipboy says the date October 31, kids dress in costume and go trick or treat around wasteland settlements (if the settlement is big enough, like megaton). Or Christmas in New Canaan? Would be nice to see in the game.

I guess it depends on where you are. I think in the first couple of years since the great war people were more focused on survival then what holiday it happened to be and over time people just stopped caring and the traditions and dates stopped being passed down to the next generation. In some of the Vaults I bet they kept up with holidays since the pipboys in the orginals would remind people what holiday it was.

Well in the wasteland, there are many traditionalists. And new holidays can always be made. If shown in the game, it can't happen yearly, because it will only happen once or twice or never in one playthrough. Holiday has to happen monthly to be part of the game.