Of lingering and gain-seeking make an end;
Think, while there’s time, how soon Death’s pyre may blaze;
And some brief folly mix with prudent ways:
At the fit hour ’tis sweet to unbend.

[Verum pone moras et studium lucri
nigrorumque memor, dum licet, ignium
misce stultitiam consiliis brevem:
dulce est desipere in loco.]

Horace (65-8 BC) Roman poet and satirist [Quintus Horacius Flaccus]

Odes [Carmina], Book 4, # 12, l. 25ff (4.12.25-28) (13 BC) [tr. Marshall (1908)]
    (Source)

Usually subtitled by translators "To Virgil" or "Invitation to Virgil." There has been great controversy amongst scholars whether the Virgil mentioned in the ode refers to the famous poet who composed the Aeneid, among other works. The two knew each other, but that Virgil died in 19 BC. Some suggest this was an older poem of Horace's, finished and inserted into this later, final volume by him.

(Source (Latin)). Alternate translations:

Think Life is short, forget thy fears,
And eager thoughts of Gain,
Short Folly mix with graver Cares,
'Tis decent sometimes to be vain.
[tr. Creech (1684)]

Come, quit those covetous thoughts, those knitted brows,
Think on the last black embers, while you may,
And be for once unwise. When time allows,
'Tis sweet the fool to play.
[tr. Conington (1872)]

But lay aside delay, and the desire of gain; and, mindful of the gloomy [funeral] flames, intermix, while you may, your grave studies with a little light gayety: it is delightful to give a loose on a proper occasion.
[tr. Smart/Buckley (1853)]

To the winds with base lucre and pale melancholy ! --
In the flames of the pyre these, alas! will be vain,
Mix your sage ruminations with glimpses of folly, --
'T is delightful at times to be somewhat insane!
[tr. Martin (1864)]

But put aside delays and care of gain,
Warned, while yet time, by the dark death-fires; mix
With thought brief thoughtlessness; to be unwise
In time and place is sweet.
[tr. Bulwer-Lytton (1870)]

Then lay aside delays, pursuit of gain, and, mindful fo the funeral pyre, intermix, while it is permitted, a temporary foolishness with thy worldly plans. There is pleasure in indulging in folly on special occasions.
[tr. Elgood (1893)]

Quick! ere the lurid death-fire's day,
Drive thou the lust of gain away!
Thy wisdom with unwisdom grace:
'Tis well to rave, in time and place.
[tr. Gladstone (1894)]

Come! a truce to delay, and the desire of gain!
And, all mindful, in time, of the dark fun'ral fires.
Mingle with your grave plans some little folly's fling,
Sweet is folly at fitting times.
[tr. Phelps (1897)]

Mingle a little folly with your wisdom; a little nonsense now and then is pleasant.
[Source (1908)]

But put aside delay and thirst for gain, and, mindful of Death’s dark fires, mingle, while thou mayst, brief folly with thy wisdom. ’Tis sweet at the fitting time to cast serious thoughts aside.
[tr. Bennett (Loeb) (1912), "The Delights of Spring"]

Quick, quit your usury. Time is fleet.
Think, while you may, of funeral flames,
And blend brief folly with your aims;
Folly, in folly's hour, is sweet.
[tr. Mills (1924)]

Then come at once and pause for breath
In chasing wealth. Remembering death
And death's dark fires, mix, while you may,
Method and madness, work and play.
Folly is sweet, well-timed.
[tr. Michie (1963)]

Don’t linger, don’t stop to be sensible,
Let a little folly mix with your wisdom,
Be aware of death’s dark fires:
Frivolity is sweet, in season.
[tr. Raffel (1983)]

And, heedful of death's black fire, consent for a while
To mix a little pleasure in with your prudence.
It's right to be foolish when the time is right.
[tr. Ferry (1997)]

Be mindful, while you may,
of black-smoked funeral pyres
and blend a bit of folly with your wisdom.
O it is sweet at the proper time
to play the fool!
[tr. Alexander (1999)]

But abolish delay, and desire for profit,
and, remembering death’s sombre flames, while you can,
mix a little brief foolishness with your wisdom:
it’s sweet sometimes to play the fool.
[tr. Kline (2015), "Spring"]

Roald Dahl had Willy Wonka use the thematically similar line "A little nonsense now and then / Is relished by the wisest men" in both his screenplay for the movie Willy Wonka and the Chocolate Factory (1971) and in the book Charlie and the Great Glass Elevator. For more information in this variant and its possible origins, see Quote Origin: A Little Nonsense Now and Then is Relished by the Wisest Men – Quote Investigator®.