Mahashivratri is the day when the Shiva Tattva touches the earth. The consciousness, the aura, or the ethereal world, which is always ten inches above the material ground, touches the earth element on the day of Mahashivratri. It is the wedding of the material with the spiritual, and that is the celebration.
Shiva is the soul (of everything) – there is no difference between the soul and Shiva. Your true nature is Shiva, and Shiva is peace, infinity, beauty, and the non-dual one. Ratri means 'to take refuge'. Shivratri is taking refuge in one’s own spirit (Shiva). It is the day that we celebrate the soul or spirit (Shiva Tattva) within ourselves.