когда рак на горе свистнет (kogdá rak na goré svístnet) — “When the crawfish whistles on the mountain”.
For more ways of saying "Never," see here. It misses a favourite of mine, though. Apparently, in ancient Rome, an expression for "never" was, "When a mule foals."
When Pigs Fly, Crayfish Whistle, and It Snows Red Snowflakes