The phrase a skeleton at the feast, or at the banquet, denotes a person or event that brings gloom or sadness to an occasion of joy or celebration.

This was originally an allusion to the practice of the ancient Egyptians, as recorded by the Greek biographer and philosopher Plutarch (circa 46-circa 120) in The Dinner of the Seven Wise Men, in Moralia:

(Loeb Classical Library edition – 1928)
The skeleton which in Egypt they are wont, with fair reason, to bring in and expose at their parties, urging the guests to remember that what it is now, they soon shall be, although it is an ungracious and unseasonable companion to be introduced at a merry-making, yet has a certain timeliness, even if it does not incline the guests to drinking and enjoyment, but rather to a mutual friendliness and affection, and if it urges upon them that life, which is short in point of time, should not be made long by evil conduct.

The earliest allusion to a skeleton at the feast that I have found is from an anonymous poem titled The Lean Man, published in the Indiana Palladium (Indiana, USA) of 17^th November 1832:

He sitteth at the dinner board,
Cadaverous and cold,
As was the veiled skeleton
At Egypt’s feast of old;
Yet worketh well his lantern-jaw,
And fast his fingers fill,—
Your fleshless bones are noted for
Their gastronomic skill.

In The Morning Post (London) of 26^th August 1840, the reviewer of Italy and the Italians by Frederick von Raumer wrote:

Sir Cracle, in fact, is always spoiling the heavy with the light, and the light with the heavy. Like the Egyptian, he will have a skeleton at every feast, and every now and then he breaks the shins of his appetite with a blow from his philosophy!

The metaphor appears in The Old Clock on the Stairs (1846), by the American poet Henry Wadsworth Longfellow (1807-82):

In that mansion used to be
Free-hearted Hospitality;
His great fires up the chimney roared;
The stranger feasted at his board;
But, like the skeleton at the feast,
That warning timepiece never ceased,—
“Forever—never!
Never—forever!”

A curious use of the image was made in The New French Constitution, an article published in The Examiner (London) of 24^th June 1848:

Another ingenious addition to the French constitution is the establishment of a tribunal to try the President, for of course he is supposed likely to turn traitor. And those who make him, are resolved to put before his eyes the process and the possibility of his being unmade—another theft from the Egyptian code, which always seated a skeleton at a feast, in order that the death’s head might check the hilarity of the living countenances.