cold day in hell
(n.) An event that will never happen. See synonym when pigs fly.
It'll be a cold day in hell before I return her calls.
a cold day in hell
the occurrence of something that will never happen
It will be a cold day in hell before we completely repair the damage done by rumors about the side effects of vaccines. After all, there will always be idiots out there. All we can do is try our best to debunk the issue.