For the ascetics, it is the day Shiva became one with Mount Kailash. He became like a mountain – absolutely still. In the yogic tradition, Shiva is not worshiped as a God, but considered as the Adi Guru, the first Guru from whom the knowledge originated. After many millennia in meditation, one day he became absolutely still. That day is Mahashivratri. All movement in him stopped and he became utterly still, so ascetics see Mahashivratri as the night of stillness.
Legends apart, why this day and night are held in such importance in the yogic traditions is because of the possibilities it presents to a spiritual seeker. Modern science has gone through many phases and arrived at a point today where they are out to prove to you that everything that you know as life, everything that you know as matter and existence, everything that you know as the cosmos and galaxies, is just one energy which manifests itself in millions of ways.
This scientific fact is an experiential reality in every yogi. The word “yogi” means one who has realized the oneness of the Existence. When I say “yoga,” I am not referring to any one particular practice or system. All longing to know the unbounded, all longing to know the oneness in the Existence is yoga. The night of Mahashivaratri offers a person an opportunity to experience this.
Source : Isha Yoga
A beautiful composition written by Prasoon Joshi and sung by Kailash Kher :