"Long day?" is a question that someone might ask if they notice that the person they are talking to seems tired, exhausted, or stressed. The phrase suggests that the person has been through a lot during the day, and that they may be feeling the effects of a demanding or challenging schedule. In this context, "long day" could refer to a day that is physically or mentally demanding, or to a day that is unusually long or eventful. By asking "long day?", the speaker is expressing concern for the other person's well-being and is inviting them to talk about their experiences or challenges. The phrase can also be used as a way of commiserating with someone who is feeling overwhelmed or exhausted, as in "I know what you mean, I've been having a long day too."