Every single month of the year has a "National _______ Month" designation. Heck, I think every day has celebrates something. June has a few celebrations, and National Fruit and Vegetable Month is one of them.
Of course, you should eat a lot of fruit and vegetables all year round, but if one month can get you going, then it's a good thing. June is usually the month when I start to eat more salads and apples and oranges anyway, as I'm sure it is with a lot of people, so it's good timing.
Here's more info and tips about the month, along with links to other sites about healthy eating. And if you insist on eating a whole pizza tonight, at least put some mushrooms and onions on it.