blue moon

基本解释

You shouldn't lose this once-in-a-blue-moon chance.
你不應該錯過這個難得的機會。
How could you miss such a once-in-a-blue-moon chance?
你怎麽能錯過這個千載難逢的機會呢？
Faced with the once-in-a-blue-moon business opportunity provided by the Olympic Games, many non-Olympic sponsors should take the strategies of non-Olympic marketing.
麪對千載難逢的奧運商機，衆多的非奧運贊助商應該採用非奧運營銷的策略。

Secretary of State James Baker -- in the political equivalent of a "blue moon" -- admitted recently that President Bush and he had erred in appeasing Iraqi strongman Saddam Hussein, a policy that contributed substantially to the present Persian Gulf crisis.
CENTERFORSECURITYPOLICY: Which side will we be on?
Maybe it's time (once in a blue moon) for a high-level bipartisan commission--televised and transparent--to review the science, the costs and the regulatory processes, taking into account the national interest.
FORBES: A No-Lame-Duck Growth Agenda
The professor and the policeman's beers are equally symbolic, Pirko says - Red Stripe with its Jamaican pedigree, and Blue Moon striving for craft beer credibility, though it's actually made by Coors.
NPR: What Brands Will Be Served At Beer Summit?

the second full moon (= the time when the moon is shaped like a complete disc) in a month, that appears around every two and a half years