Eating produce in season is not just good for your health, it's also good for the environment. Having trucks haul fruits and veggies cross-country when they are not in season requires a lot of extras that aren't good for you, including what goes in to helping a truck travel cross-country as well as the gasses that are used to keep produce fresh longer.
Finding a reliable, local source is your best bet and knowing what's in season when, really helps your bottom line because even if you're buying organic, if its in season, it will be cheaper than imported/shipped items that are not.
Here's an at-a-glance list I've created to help you know what's good each season! Now....go eat your strawberries! But get them organically if you can...they are on the dirty dozen list!