Traditionally, a luncheon is a lunch that takes an eon.

Judith Martin (b. 1938) American author, journalist, etiquette expert [a.k.a. Miss Manners]

“Miss Manners,” syndicated column (1982-04-18)
    (Source)

Reprinted in Miss Manners' Guide for the Turn-of-the-Millennium, Part 6 "Genuine Social Life," "Social Occasions" (1989). Often incorrectly attributed to Miss Manners' Guide to Excruciatingly Correct Behavior (1983).