Some fruits and vegetables are less expensive when they are “in season” -- meaning they are just ripe from the fields. When the supply for a food like sweet corn in the summer, apples and squash in the fall is plentiful, the cost goes down.
A trip the local farmers' market will give you clues about what is in season in your area. Produce in grocery stores is a less reliable indicator of food is in season because most fresh fruits and vegetables are available year-round.
Here are a couple of lists of seasonal fruits and vegetables. The first one is based on Iowa and the other is a national list.

When it comes to good nutrition, all forms of fruits and vegetables are nutritious -- fresh, frozen, canned, dried, and 100% juice.
- Most frozen and canned foods are processed within hours of harvest, so their flavor and nutritional value are preserved.
- Studies show that recipes prepared with canned foods had similar nutritional values to those prepared with fresh or frozen ingredients.
- Frozen foods also require little preparation-washing and slicing is already done.