blue moon

基本解释

You shouldn't lose this once-in-a-blue-moon chance.
你不应该错过这个难得的机会。
How could you miss such a once-in-a-blue-moon chance?
你怎么能错过这个千载难逢的机会呢？
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.
面对千载难逢的奥运商机，众多的非奥运赞助商应该采用非奥运营销的策略。

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