Why is the ninth month called September?
1748 (in Chesterfield's "Letters"), but rare before c.1800 as an English institution; originally a fashionable pot-luck social affair, not necessarily out of doors; from French piquenique (1690s), perhaps a reduplication of piquer "to pick, peck," from Old French (see pike (n.2)), or the second element may be nique "worthless thing," from a Germanic source. Figurative sense of "something easy" is from 1886. Picnic table recorded from 1926, originally a folding table.
"go on a picnic," 1842, from picnic (n.). Related: Picnicked; picnicking. The -k- is inserted to preserve the "k" sound of -c- before a suffix beginning in -i-, -y-, or -e- (cf. traffic/trafficking, panic/panicky, shellac/shellacked).