● This is a once in a lifetime event.
（これは一生に一度のできごとだ）
once in a lifetime で、「一生に一度のこと」の意味です
日本語でもそう言いますが、英語でも同じように言います
It's once in a lifetime.
That can happen once in a lifetime.
It was really once in a lifetime.
He said, " This is a once in a lifetime opportunity."
今週から秋でしょうかね
Take care!