"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 :)

It's not something that happens every day, it's something that happens just once in life.

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."