* Time: A day is a unit of time, typically 24 hours.
* Period: A day can refer to a specific period of time, like "a day in the park" or "the day after Christmas".
* Date: A day can be a specific date on the calendar, like "the first day of January".
* Part of a cycle: A day can be a part of a larger cycle, like "a day in the life of..." or "a day at the beach".
Therefore, "day" isn't limited to just one noun category. It can fit into several depending on how it's being used in a sentence.