it'll be a frosty Friday
It is unlikely or impossible (that something 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.
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.