Think of it this way: You tell your friends how sweet/cute/funny/angelic your little baby is. The day comes for you to drive an hour to your friend's home to show off your little bundle. You've driven during her nap time, but she doesn't sleep well in the car. Unfortunately, by the time you get there, the happy, adorable child you've bragged about is fussing, crying, and unwilling to crack a smile, let alone say "meow" when you ask her what sound a kitty makes. Fruits and vegetables are a lot like a baby that way; they sometimes need to be allowed to rest without disruption and will only travel so far before their ability to perform is compromised. The moral of the story: Respect the needs of your produce, and it won't just meow for you; it'll sing the entire soundtrack to "The Lion King." It doesn't mean that if you get your oranges from Florida and you live in Virginia they're not going to taste good. It just means they won't taste as good. Just do the best you can, and definitely at least try to buy them when they're at their peak, even if they have to travel a bit to get to you.
So, what's in season? I'm sorry to say it really depends quite a bit on where you're from. Still, I think it's helpful to at least have an idea, so I've created a chart to get you started. Unless you're from Florida or Southern California where a bounty of produce seems to fall from the sky year-round, this chart should give you a decent gander at what will make your dishes perform for your guests.
Also, I came across these clips on foodnetwork.com recently which give some really great suggestions for how to select and cook different types of Winter produce. Just to give you an idea: I learned how to choose a mushroom based on its stem and how to clean it properly. I also learned that the leaves of carrots can be used as an herb! (Forgive the commercial at the start of each.. Sorry!)