Shiva is Virupaksha -- one who is formless yet who sees all. We know there's air all around us; we can feel it. But what if the air also starts feeling us? Space is all around us, we identify space. But what if space also recognizes and feels our presence? This happens. Only, we don't know it. Scientists know this and they call it the theory of relativity. The one who sees and that which is seen are both affected when seen. The formless Divine, Shiva, is all around you and is seeing you. He is the seer, sight and the seen. To wake-up and experience this Shiva tattva is Shivratri.
Usually when there is celebration, awareness is lost. Deep rest in celebration with awareness is Shivratri. When you face some problem, you become aware and alert. We are at rest when everything is well; on Shivratri we rest with awareness. It is said that a yogi remains awake when everybody else is sleeping. For a yogi, everyday is Shivratri.