Probably, but it could also mean a day when one's efforts are ineffective. It would depend on context.

Phil makes a good point. It could mean that one is feeling "down" as in ineffective or in a depressed mood. As explained by him, it depends on the context.

Yes it does! I'm taking a "down day." I'm kicking back, relaxing, taking it easy, sipping on a cold one.

OK! Thank you!