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being on duty; person on duty; one's turn
今日は私が掃除当番の日だったんだよ。
Today was my day for cleaning duty.
申し訳ないけど当番を交代してもらえませんか。
I'm sorry, but could I get you to swap duty shifts with me?
夜勤の当番は体力的にきつかった。
The night shift duty was physically tough.