It can also be used to mean that people are behaving strangely, because they are very bored, because it's a slow day. For example, if a major newspaper runs a front-page article on the vitamin C content of organically-grown lemons vs. regular lemons, then you might comment "it's a slow day at the newspaper office"... because they obviously they have nothing more important to report. In a way, it's used to mean "don't they have anything better to do?"

"It's a slow day."= business is down or you are not as busy as usual

boring day with not much to do, so time goes by slowly

Propably it means that the day is very lazy. Wen we have slow day we don't want to do anything. We are lazy and everything pass slowly that day.

It means: It's not a fast day.