There are numerous locutions to express this idea; as-
At the coming of the Coqueligrues (Rabelais: Pantagruel). At the Latter Lammas. (See Lammas.)
On the Greek Calends (q.v.)
In the reign of Queen Dick. (See Dick.)
On St. Tib's'Eve. (See Tibs Eve.)
In a month of five Sundays. (In) la semaine des trois jeudis.
When two Fridays come together
When three Sundays come together
When Dover and Calais meet. (See Dover.)
When Dudnian and Ramehead meet. (See Dunman.)
When the world grows honest.
When the Yellow River runs clear.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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