It's totally subjective. From your perspective, it's the start of a new day but it just exists in your mind. I'm reality, the closest thing to a day is the sun cycle that from our perspective looks like the sun rising in the east in the morning (another man-made construct), and setting in the west in the evening. But even that doesn't really exist because the sun doesn't actually rise and set. it just looks that way, again from our perspective, when in fact it's just how we perceive the perpetual rotation of the earth.