It's not the day which changes, only potentially the individual perceiving it. While it may also be true that the perception of time's passage is relative, what I'm referring to is the actual change of a human from one day to the next, I believe that's the significance. I question our intuitive presumption and feeling of continuity of consciousness over the course of our lives, or perhaps I once was preoccupied with doing so.
However I've now come to the conclusion that my belief in a unifying identity, a force which projects itself onto the past and future as we look upon it is what presents itself as the "continuity". The past is unknowable, while it may appear as if this is not the case, you cannot know the past as you cannot ever truly experience it. You can only see the present, and the future is obviously uncertain to us. However the reason why we have the capacity to project emotions and ideas upon the past, it's simply because what we're seeing is not the true past at all, but merely our own idea of it, a construct purely of our own present minds. That combined with our aspirations which reach into the future form the core of our individual continuity. We continue on throughout our lives believing that we were always there, and this is actually correct, at least in any discernible manner, since we were always present in every moment which we perceive.