Many singles plan their dates at least a few days ahead, but it could become an issue when they get up in the morning. Rather than being ready to tackle the world, they could find they are feeling under the weather as they go through their normal routine. A lack of energy and interest could be telling them that canceling their date for that evening would be a good idea, but they generally balance it against whether or not they were looking forward to going out with their date.
It can be a big decision for a single person to forego a date, and they might decide it would be best to carry on. It could be they have just met the person and wish to avoid offending them, or they might have tickets to a special event with a person they have been dating for a few months. Some dates might not appreciate being with someone feeling ill, but others could see it as a way to cement their relationship when they take the person home early and make sure they are okay.