Food historians tell us picnics evolved from the elaborate traditions of outdoor feasts enjoyed by the wealthy. Medieval hunting feasts & Renaissance-era country banquets probably were the earliest picnics.
1737 Carle or Charles-André van Loo (1705-1765) After the Hunt
"Picnic. Originally, A fashionable social entertainment in which each person present contributed a share of the provisions." The OED traces the oldest print evidence of the word picnic in the English language to 1748. The word was known in France, Germany, and Sweden prior to becoming an English institution.
---Oxford English Dictionary [Clarendon Press:Oxford], 2nd edition, Volume XI (p. 779)
"The earliest picnics in England were medieval hunting feasts. Hunting conventions were established in the 14C, and the feast before the chase assumed a special importance. Gaston de Foiz, in a work entitled Le Livre de chasse (1387), gives a detailed description of such an event in France. As social habits in 14C England were similar to those in medieval France, it is safe to assume that picnics were more or less the same."
---Oxford Companion to Food, Alan Davidson [Oxford University Press:Oxford] 1999 (p. 602)
1737 Nicolas Lancret (1690-1743) The Hunting Party Meal
"The French might have invented the word "picnic," pique nique being found earlier than "pic nic." It originally referred to a dinner, usually eaten indoors, to which everyone present had contributed some food, and possible also a fee to attend. The ancient Greek "eranos," the French "moungetade" described earlier, or modern "pot luck" suppers are versions of this type of mealtime organization. ...Picnics derive, also, from the decorous yet comparatively informal 16C "banquets"...whichh frequently took place out of doors."
---The Rituals of Dinner: The Origins, Evolutions, Eccentricities and Meaning of Table Manners, Margaret Visser [Penguin:New York] 1991 (p. 150-1)
1740 Nicolas Lancret (1690-1743) Picnic after the hunt
"Picnic. An informal meal in which everyone pays his share or brings his own dish,' according to the Littre dictionary. That was probably the original meaning of the word, which is probably of French origin (the French piquer means to pick at food; nique means something small of no value.) The word was accepted by the Academie francaise in 1740 and thereafter became a universally accepted word in many languages. From the informal picnic, the outdoor feast developed...Weekend shooting parties and sporting events were occasions for grand picnics, with extensive menus and elaborate presentation."
---Larousse Gastronomique, completely updated and revised edition [Clarkson Potter:New York] 2001 (p. 883)