it'll be a frosty Friday

it'll be a frosty Friday

It is unlikely or impossible that it will ever occur. Sometimes followed by "in Hell," or "in July" or other hot places or times to add more emphasis. It'll be a frosty Friday when I sign over the lease of my land to you. It will be a frosty Friday in July before the two parties agree to anything.
See also: Friday, frosty

it'll be a frosty Friday (in July)

used to indicate that something is very unlikely to happen. Canadian informal
1990 Walter Stewart Right Church, Wrong Pew It would be a frosty Friday in the middle of July before he would discuss personal affairs with the press.
See also: Friday, frosty