Greek calends

English

Calque of Latin ad kalendās Graecās; unlike the Roman calendar, the Greek calendar had no calends.

Greek calends pl (plural only)

(idiomatic) A time that never occurs; never; when pigs fly.
- 1923, [François Béroalde de Verville], “Origin of the Decretals”, in Arthur Machen, transl., Fantastic Tales or The Way to Attain—a Book Full of Pantagruelism Now Done for the First Time in English, Carbonnek [i.e., London]: Privately printed, →OCLC, page 98:
  Blockheads, friends of my heart and liver, cousins of my tripe, are you ignorant that this symposium is as authentic as any of those tales of the Greek Calends, which you swallow and digest so easily, [...]?
- 1950 January 12, C[live] S[taples] Lewis, “Letters: 1950 [To Sister Penelope CSMV (BOD)]”, in Walter Hooper, editor, The Collected Letters of C. S. Lewis, volumes III (Narnia, Cambridge and Joy 1950–1963), New York, N.Y.: HarperSanFrancisco, HarperCollins, →ISBN, pages 5–6:
  My book with Professor [John Ronald Reuel] Tolkien – any book in collaboration with that great but dilatory and unmethodical man – is dated, I fear, to appear on the Greek Kalends!

Used after a preposition such as at, on, or till.

a time that never occurs

never (translations include the leading preposition)