A picnic is a meal taken outdoors (al fresco) as part of an excursion, especially in scenic surroundings, such as a park, lakeside, or other place affording an interesting view, or else in conjunction with a public event such as preceding an open-air theater performance, and usually in summer or spring. It is different from other meals because it requires free time to leave home.

Historically, in Europe, the idea of a meal that was jointly contributed to and enjoyed out-of-doors was essential to picnic from the early 19th century.

Picnickers like to sit on the ground on a rug or blanket. Picnics can be informal with throwaway plates or formal with silver cutlery and crystal wine glasses. Tables and chairs may be used, but this is less common.

Outdoor games or other forms of entertainment are common at large picnics. In public parks, a picnic area generally includes picnic tables and possibly built-in barbecue grills, water faucets (taps), garbage (rubbish) containers, restrooms (toilets), and gazebos (shelters).

Some picnics are a potluck, where each person contributes a dish for all to share. The food eaten is rarely hot, instead taking the form of sandwiches, finger food, fresh fruit, salad, and cold meats. It can be accompanied by chilled wine, champagne, or soft drinks.

History
The practice of an elegant meal eaten out-of-doors, rather than an agricultural worker's mid-day meal in a field, was connected with respite from hunting from the Middle Ages; the excuse for the pleasurable outing of 1723 in François Lemoyne's painting (illustration) is still offered in the context of a hunt. In it, a white cloth can be seen, and on it wine, bread, and roast chicken.

While these outdoor meals could be called picnics there are, according to Levy, reasons not to do so. 'The English', he claims, 'left the hunter's meal unnamed until after 1806, when they began calling almost any alfresco meal a picnic'. The French, Levy goes on to say, 'refrained from calling anything outdoors a pique-nique until the English virtually made the word their own, and only afterward did they acknowledge that a picnic might be enjoyed outdoors instead of indoors'.

Pic Nic Society
The French Revolution popularized the picnic across the world. French aristocrats fled to other Western countries, bringing their picnicking traditions with them.

In 1802, a fashionable group of over 200 aristocratic Londoners formed the Pic Nic Society. The members were Francophiles or may have been French, who flaunted their love for all things French when the wars with France lulled between 1801 and 1830. Food historian Polly Russell however suggests that the Pic Nic Society lasted until 1850. The group intended to offer theatrical entertainment and lavish meals followed by gambling. Members met in hired rooms in Tottenham Street. There was no kitchen so all food had to be made elsewhere. Each member was expected to provide a share of the entertainment and of the refreshments, with no one particular host.

Source
Wikipedia