Sometime in 1798-1799, the date is unclear; Woodsworth describes a picnic in The Prelude.  He does not refer to this adventure as a picnic because the word was unknown to him at this time. He probably became familiar with the word a decade later because John Wilson invited him and his sister, Dorothy, to picnic on Grasmere. * However, in all of his poetry and prose, Wordsworth never uses the word picnic.

Because The Prelude went through four revisions, it’s interesting to see how Wordsworth shifted his emphasis and gradually condensed his idea of picnicking. In 1799 the text is explicit about picnicking. While rowing on the lake, he and his friends stop for a “delicate meal” purchased from a nearby home. 
An old Hall was near,
G
rotesque and beautiful, its gavel end
And huge round chimneys to the top o’ergrown
With fields of ivy. Thither we repaired,
‘Twas even a custom with us, to the shore
A
nd to that cool piazza. They who dwelt
In the neglected mansion-house supplied
Fresh butter, tea-kettle, and earthenware,
And chafing-dish with smoking coals, and so
Beneath the trees we sat in our small boat
And in the covert eat our delicate meal
Upon the calm smooth lake.  (ll. 146-157)      

The adventure on Coniston is condensed in 1805, and the episode is now referred to as an “excursion.” Meals are now “rustic dinners:”
Excursions far away among the hills,
Hence rustic dinners on the cool green ground.
Or in the woods, or near a river side,
Or by some shady fountain, while soft airs
Among the leaves were stirring, and the sun
Unfelt, shone sweetly round us in our joy. (ll. 85-93)

“Excursion” is dropped in the final text of 1850, but not “rustic dinner:”
But now to school
F
rom the half-yearly holidays returned,
We came with weightier purses, that sufficed
To furnish treats more costly than the Dame
Of the old gray stone, from her scant board, supplied.
Hence rustic dinners on the cool green-ground,
Or in the woods, or by a river side,
Or shady fountains, while among the leaves
Soft airs were stirring, and the mid-day sun
Unfelt shone brightly round us in our joy. (ll.85-94)

Featured Image: J.M.W. Turner. Coniston Fells, Coniston Old Man (1798). Wordsworth does not fully explain how rugged the landscape was in his time.

See William Wordsworth. The Prelude: The Four Texts (1798, 1799,1805, 1850). Edited by Jonathan Wordsworth. London: Penguin Books, 1995; 1850 text; https://en.wikisource.org/wiki/The_Prelude_(Wordsworth) 1850

*Dorothy Woodsworth in elsewhere in PicnicWit.com. * In 1808, Dorothy Wordsworth attended a John Wilson (aka Christopher North) picnic on Grasmere. The word a novelty and she wrote to a friend asking about its etymology. Because Dorothy and William discussed just about everything, they must have discussed picnic and then forgotten about it. When Dorothy picnicked on the summit of Scaw Fell in 1818, she does not use the word.