Ellipsis of when the cat's away, the mice will play.

simple past tense of know someone when

Alternative form of when all is said and done

(archaic) One should take immediate advantage of an opportunity that may not be available later.

(uncommon) Alternative form of when in Rome, do as the Romans do

(figuratively, idiomatic) When the pressure is on; when the situation is critical or urgent; when the time has come for action, even if it is difficult.

(idiomatic, slang, informal, hyperbolic) Never; not in this lifetime or a time in the unforeseeably distant future; not a chance.

(idiomatic) in the end; ultimately

(idiomatic) To make things worse for someone who is going through a difficult time.

Warning, often in the form of a sign, that people should pay attention when walking on a wet and slippery floor not to fall down and get injured.