The most wasted of all days is that on which one has not laughed.

[La plus perdue de toutes les journées est celle où l’on n’a pas ri.]

Nicolas Chamfort (1741-1794) French writer, epigrammist (b. Nicolas-Sébastien Roch)

Products of Perfected Civilization [Produits de la Civilisation Perfectionée], Part 1 “Maxims and Thoughts [Maximes et Pensées],” ch. 1, ¶ 80 (1795) [tr. Morley (1887)]
    (Source)

Often attributed to a more contemporary comedian (Groucho Marx, Charlie Chaplin) or writers such as Ben Burroughs, Grigori Alexandrov. It is arguably a clear enough sentiment that others might reinvent it.

(Source (French)). Alternate translations:

The most lost of all days, is that in which we have not laughed.
[Source (1803)]

The most completely lost of all days is that on which one has not laughed.
[Source (1891)]

The worst wasted of all days is that during which one has not laughed.
[tr. Hutchinson (1902), "The Cynic's Breviary"]

Of all days, the day on which one has not laughed is the one most surely wasted.
[tr. Mathers (1926)]

That of all days is the most completely wasted in which one did not once laugh.
[tr. Merwin (1969)]

The day that we have most lost is the one on which we have not laughed.
[Siniscalchi (1994)]

Other versions:

• "A day without laughter is a day wasted." [attr. to Chaplin]
• "The most lost of all days is that in which one has not laughed."
• "The most wasted day of all is that in which we have not laughed."

More history of the quotation: A Day Without Laughter is a Day Wasted – Quote Investigator®.