值日 means 'to be on duty for the day', commonly used in schools, offices, or group settings where people take turns handling daily tasks. As a separable verb, you can insert time expressions or measure words between 值 and 日 — for example, 值过日 ('have been on duty') or 值了三天日 ('was on duty for three days'). The person you're standing in for or the specific duty is usually clear from context.