1"The other day" or "a few days ago" — refers to a recent, unspecified date in the near past. Used in both casual speech and formal writing.
先日はお忙しいところありがとうございました。
Thank you again for your time the other day.
先日、友達と新しいレストランに行きました。
The other day I went to a new restaurant with a friend.
先日聞いた話ですが、来月イベントがあるそうです。
I heard the other day that there's going to be an event next month.