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now that you mention it; come to think of it; that reminds me; on that subject; speaking of which
そう言えば、明日は休みでしたね。
Come to think of it, tomorrow is a holiday.
そう言えば、彼に会いませんでしたか。
Speaking of which, didn't you see him?
そう言えば、来週は祝日だったね。
Come to think of it, next week is a holiday, isn't it?