It is a picnic (modern meaning = informal meal held in the open air) held at harvest-time (European agricultural tradition = the time when wheat and other cereals are cut, in late summer).
Originally a harvest picnic was informal: all the harvesters took their food into the field to eat and drink together* when the work was done (original meaning of picnic = a meal where each person brings their own part, to share with others). Then it became formalised, perhaps organised by the landowner or by the church. It has various names: harvest festival, harvest home, harvest supper... Food, drink and music are usual ingredients; games and dancing are common.
[* other activities too: see Laurie Lee's autobiography Cider with Rosie, where he has his first sexual experience under a hay-wagon.]