The Movie: Young Frankenstein, 1974 (Gene Wilder and Mel Brooks, screenwriters; Mel Brooks, dir.)
Who says it: Marty Feldman as Igor (Eye-gor), Frankenstein’s assistant
The context: Igor and Dr. Frankenstein are robbing a grave, and Frankenstein says it’s a filthy job. Immediately after Igor says this, it starts raining.
How to use it: In acceptance of the fact that whatever you’re doing is about to get worse. It's most effective in a Cockney accent, but don't try it if you can't really do one.
An inexplicable cheerfulness takes over once you realize there's no way out of a miserable situation. And it's hardly likely to rain here today.