There are two concepts of spiritual illumination. One is that Shakti awakens, goes up the sushumna nadi and unites with Shiva in the sahasrara chakra. Shiva represents the higher cosmic consciousness and Shakti represents evolution of energy. This is the concept which kundalini yoga is based upon.
The other concept is that the consciousness goes to meet Shakti and this is Shivaratri. The concept of Shivaratri is the awakening of consciousness at the material level of existence and uniting with Shakti at a higher point in evolution. Therefore the word used is ratri meaning 'dark night'. What are the night and day of consciousness? When the individual experiences existence, the objective reality all around him, that is the day of consciousness. The night of consciousness is when the consciousness is all alone and no objective experience takes place. You don't hear, see, feel or know anything. Time, space and objectivity - the three qualities of mind - fall flat. Consciousness alone remains. That is the dark night of the soul, the stage just before illumination. So Shivaratri is a symbol of the spiritual state of samadhi. But for us, Shivaratri means the state preceding samadhi - illumination.
The story goes like this: Shiva decided to marry Parvati, daughter of the Himalayas, who lived up in the snow peaks. Shiva lived in the forest He was the master, guru and controller of ghosts and demons, and when he went to marry Parvati, they all accompanied him. Some had one eye in the back of their head, some had no eyes, or eyes in their belly. Some had only one ear, others had huge elephant ears or only holes for ears. Some walked on one leg, others on three. So Shiva proceeded with his strange marriage procession. As he neared the Himalayan kingdom where Parvati lived, her family sent out a reception party to meet and escort them to Parvati's house. However, when the reception party caught sight of Shiva and his strange companions, instead of greeting their guests respectfully and leading them back to Parvati's house, they took to their heels and ran for dear life.
As soon as they returned to Parvati's house, they related what they had seen in awe and horror. 'Oh, he is terrible! The son-in-law has come riding on a bull. He is naked and his body is smeared with ash. He has snakes all over him and his companions are the most hideous, fearful people.'
Parvati's mother was so upset. How could she ever accept such a horrible son-in-law? But Parvati remained calm and resolute. 'I shall marry him', she told her mother, 'I won't be deterred'. The moment Lord Shiva's procession entered the Himalayan kingdom, he and all his funny companions turned into dazzling, divine beings with beautiful faces, fine clothes, glittering necklaces, fragrant flowers and all sorts of things. The demons changed into lovely people; everything was transformed in the twinkling of an eye, and so the marriage took place.
Shiva is symbolic of consciousness. For the individual, consciousness is moving higher and higher towards Shakti. It moves along with all our instincts and animal propensities, with all that we are. Even as you practice yoga, everything is still with you - fear, anger, passion, worries, anxiety - you are moving with all your companions. Your soul is also evolving, progressing, along with all your companions. But there comes a point in spiritual life when all your companions, instinctive life and animal propensities, are transformed and the same instinct becomes intuition.
During the course of spiritual evolution, you try many tunes and fail - you go to the church for the wedding but when you get to the door you find that the bride is not there and you have to return home disappointed. You have inspirations, you may catch a glimpse of the higher state but it is not complete. When the time comes and the transformation occurs, the ugly companions with noses in their stomachs and eyes in the backs of their heads turn into divine attendants with suits and ties! The horrifying aspects of your personality become your ornaments, your helpers.
That is how the story of Shivaratri goes. Parvati symbolizes the higher energy; she also symbolizes the kundalini shakti in tantra. The divine union which takes piece when Shiva comes to meet Shakti represents enlightenment in absolute darkness.