You don’t need someone to tell you they’re having a bad day. Their one-word answers already did.

People signal long before they explain, and they usually do it by subtraction. The replies get shorter, the jokes stop, they sit slightly outside the group. Nobody announces a bad day because saying it out loud makes it bigger, so it leaks instead. You already have the information before anyone hands it to you.

Do not force the conversation. Ask something easy, say you are around, then let them take it or leave it. Sitting next to someone without needing them to perform being fine is often the whole thing they needed.