千載一遇
sen.zai.ichi.guu

Literally: thousand – ride/publish/get on – one – encounter/receive (guests)

Alternately: A once-in-a-lifetime opportunity. The kind of chance that only comes along once in a thousand years.

Notes: 千載 refers to a thousand years; 一遇 is a single 遭遇, “encounter.” This saying doesn’t refer to rare occurrences or coincidentally meeting someone; it refers to an unexpected opportunity to gain some profit or benefit.

For some reason, this was one of the top results when I did an image search. From this random blog post about fishing.