"It's not every day, but something which happens only once in a lifetime."
or
"It's not something which happens every day, but only once in a lifetime."
I'm not an expert about Spanish though :)
2014년 9월 19일
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It's not something that happens every day, it's something that happens just once in life.
2014년 9월 19일
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This seems a more natural translation to me. (Native English/ Fluent Spanish)
"It's not something that happens everyday, but just once in a life time."