Another day; at a later date; used to indicate doing something later rather than now.
詳しい日程は後日メールでご連絡します。
We will inform you of the detailed schedule by email at a later date.
その件は忙しいので後日改めて相談しましょう。
We're busy now, so let's discuss that matter again another day.
書類は後日持参します。
I will bring the documents on another day.