In a post on the official forums, SimGuruGraham explained more about the Festival Lots and worlds:
Hey everyone, SimGuruSarah sent me over here because people were curious about seasonal festivals in upcoming worlds.
So here’s how it works…
We built unique lots for every single world from the expansions and the store that we’ve released so far, and once Seasons comes out all of that will be immediately available to you.
Having those festivals in the game for each world does require a little bit of coding though, and auto placing lots on worlds released by the store isn’t something we currently have the tech to support – it needs to be built in for a world after it’s released.
That being said, when Seasons comes out you’ll already have twelve different festivals to choose from that you can place in newly released worlds, allowing you to select from the one you feel fits best. In addition, the festivals are fully customizable, and I’m positive our talented building community will be creating festivals of their own that will fit perfectly into various worlds 🙂