RichThesaurus
when Hell freezes over
vs
when two Sundays come together
when Hell freezes over
adv
Never; not in this lifetime or a time in the unforeseeably distant future; not a chance.
when two Sundays come together
adv
never
How often have the words
when Hell freezes over
and
when two Sundays come together
occurred in a corpus of books?
(source:
Google Ngram Viewer
)