Trick or treating very much is and I love it. Children AND Teens alike will come visit your sims apartment or home and knock on the door. You can give them candy or give them a scare. Though kids didn't start coming til after school time for me in my game which made sense. They'll come in costumes too.
You can also place a candy bowl outside on a table and they'll grab it if your sim happens to be too busy or just not at home. It's pretty cool to throw a spooky party at your sims home and you get trick or treaters coming to the door as well too.
Haven't played with easter just yet.
You can buy fireworks in San Myshuno at the stalls at cons. Though I THINK you can also order them on the computer as well.