GRAPES! They're everywhere, so it's no surprise that people celebrate wine-making, and deities connected to the growth of the vine. Whether you see him as Bacco, Bacchus, Dionysus, the Green Man, or some other vegetative god, the god of the vine is a key archetype in harvest celebrations.
The Greek Dionysus was representative of the grapes in the vineyards, and of course the wine that they created. As such, he gained a bit of a reputation as a party-hardy kind of god, and his followers were typically seen as a debauched and drunken lot. However, before he was a party god, Dionysus was originally a god of trees and the forest. He was often portrayed with leaves growing out of his face, similar to later depictions of the Green Man. Farmers offered prayers to Dionysus to make their orchards grow, and he is often credited with the invention of the plow. In Roman legend, Bacchus stepped in for Dionysus, and earned the title of party God.
In fact, a drunken orgy is still called a bacchanalia, and for good reason. Devotees of Bacchus whipped themselves into a frenzy of intoxication, and in the spring Roman women attended secret ceremonies in his name. Bacchus was associated with fertility, wine and grapes, as well as sexual free-for-alls. Although Bacchus is often linked with Beltane and the greening of spring, because of his connection to wine and grapes he is also a deity of the harvest.
In medieval times, the image of the Green Man appeared. He is typically a male face peering out from the leaves, surrounded by ivy or grapes. Tales of the Green Man have overlapped through time, so that in his many aspects he is also Puck of the midsummer forest, Herne the Hunter, Cernunnos, the Oak King, John Barleycorn, Jack in the Green, and even Robin Hood. The spirit of the Green Man is everywhere in nature at the time of the harvest -- as leaves fall down around you outside, imagine the Green Man laughing at you from his hiding place within the woods!