In order to really enjoy the Seasons expansion pack, you need to have the festival grounds set in your town. Part of the reason why this is complicated is that Seasons is the first expansion pack to be released without a new town included. And while they released the new Monte Vista town concurrently with Seasons, Monte Vista is not Seasons-ready, in the sense of having its own festival grounds. Confusing? Yes. Surprising? Perhaps not.
Having played with Seasons for two weeks now, I continue to be blown away by what a big difference it makes in the game. This expansion pack, more than any other, truly changes the experience of playing The Sims 3.
I finally picked up a copy of Seasons (I found it on sale for half off - that never happens!) One of the first things I played with was the Weather Stone.