I'd guess they're something like a picnic, but with hayrides or Maypoles or other reminders of old agricultural festivals. There are some tips about how to plan one in Florence Howe Hall's A Handbook of Hospitality for Town and Country (which includes some quaintly troubling advice about bringing along a humorist for those times when the Irish cook gets the sandwich orders wrong).
For your reading pleasure, here's a bit more from pages 26 through 28 of Florence Howe Hall's handbook:
"The uncertainty of the weather is always the great obstacle in the path of the country hostess, who is planning some out-of-door frolic. The telephone has now come to her assistance, even in remote spots, so that she may invite her guests or postpone the affair, at short notice. A friendly barn or a sheltered verandah, sometimes proves a pleasant refuge from a sudden thundershower. A hospitable country gentleman had colored panes of glass set around his parlor windows, so that his friends, looking through these, could behold the tender green of Spring, the yellow tones of Midsummer, or the red hues of Autumn, even when skies were dull and rain pouring down.
The passing of the scythe has divested haying time of some of its old picturesqueness, but the scent of the freshly cut grass and the beauty of the fields still remain. Truly delightful is a supper party in the meadows, where the new-mown hay is piled in little fragrant cocks. The sun sinking in the west, perhaps a slender new moon hanging timidly in the sky, like a bashful maiden, the frogs whistling in the marshes, the delicious odor of the fresh hay, all combine to charm the senses. To drink lemonade through the slender straws of drying grass is a part of the pleasant programme. A pitcher of the old-time beverage — molasses, ginger and water — should certainly be provided, as it is supposed to be very satisfying to thirst. Doughnuts and pie or sandwiches and cake, may complete the simple supper. A good story-teller is a pleasant addition to a hay-stack party, although he must not feel hurt if the younger guests stroll away in couples. A small hay-cock makes a very comfortable back for two persons to lean against."