@bluedolphin-- Well, that is a harvest festival. It's just a festival concentrating on a specific fruit or vegetable. And I think these are more common in the US. People like to put together festivals for a food grown locally and abundantly. In California, for example, there is a zucchini festival and in Iowa, a corn festival. But other local foods and treats are often featured too. I'd definitely consider these harvest festivals. There is no specific definition of what one should be like.
I think though, harvest festivals were more common in the old days because farming was generally more difficult and less productive. So when people had a particularly good harvest, they felt very blessed and wanted to express their happiness and gratefulness. A festival was a great way for a community to rejoice and share in that way. Now, thankfully production is greater because of advanced farming techniques, fertilizers, etc. So festivals are, as you said, more of a reason to share the excess produce that a family can't consume alone.