From IGN.com

EA has just announced the next Sims expansion, The Sims 3: Seasons. In case you haven’t guessed it, this expansion pack adds the oft-requested seasons to the world of The Sims. Now your Sims will have a chance to interact with changing weather, as well as participate in various festivities associated with the seasons.
Sims 3 Associate Producer Ryan Vaughan feels that the expansion adds more immersion to The Sims, stating:
As much as I love summer, the fact that every day in The Sims 3 has the same exact weather can take away from the realism of the open world. With The Sims 3 Seasons, that is completely changed. Having both seasons and weather transitioning naturally during play adds a whole new dynamic to not only the realism of the open world, but to the gameplay as well. You can let your Sims swim in the ocean on a hot summer day, bob for apples among the autumn leaves, test their snowboarding skill on the half pipe, or welcome spring with a colorful umbrella and a walk in the rain! With all new activities to keep your Sims busy year-round, and an all new outerwear category in Create A Sim, seasons and weather create a brand new element for players to interact with.
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The weather can also directly affect the health of your Sims, giving them a tan if they stay out in the sun, getting them struck by lightning during a storm or even making them catch a cold or possibly freeze to death. Of course your Sims can also dress accordingly, with new fashion options including snow clothes, raincoats, and Halloween costumes. You can also give your Sims new traits like Loves the Heat or Loves the Cold to make them more impervious to various temperature extremes.
Seasons are still under your control, though. Vaughan says, “The length of a season — even the mere existence of a season — is fully customizable by the player,” adding that there’s a new menu to tweak the settings to your liking.
Some activities will only be available during certain seasons. Vaughan clarifies, stating, “For instance, having a snowball fight in the middle of summer just doesn’t make sense. However, the snow ball fight interaction is replaced by a water balloon fight during the times of year where there’s no snow on the ground. There’s always plenty of new features and content available despite the current weather.” When IGN asked whether or not the seasons affected the new swimming feature, which allows your Sims to hop into the ocean, Vaughan said, “Swimming in the ocean can be done year round,” though, “Sims who decide to take a dip during a particularly cold day may receive the Polar Bear Club moodlet, giving them a temporary immunity to the cold.”
Likely in an effort to be as inclusive to as much of the gigantic Sims audience as possible, the holidays included in The Sims 3: Seasons won’t be named after those in real-life. “The world of The Sims is a lot like ours, but with some pretty funny and amusing differences. Sims can go trick-or-treating on Spooky Day, or host a gift exchange party on Snow Flake Day. A good old-fashioned Hot Dog Eating Contest on Leisure Day happens to be one of my favorites,” says Vaughan.
Other new features include UFOs, complete with alien abductions, and ice furniture. Various items such as barstools or love seats made of ice will now be purchasable. Additionally, your Sims will also have new lifetime wishes associated with participating in various seasonal activities.