The authentic days for worshipping Lord Shiva are Mondays, Chaturdashi tithi of fortnights of every month, and mainly Shivaratri. When all other Gods are worshipped during the daytime, Lord Shiva is worshipped at night.
As in scriptures:
People believe that on this day, Lord Shiva appeared on earth. Some people also believe that there was a fight between the Gods and the Devils which resulted in the churning of the sea. During the process, there evolved a pot of poison which Lord Shiva drank and saved the Earth from great danger. Consuming this poison made Him unconscious. Until He regained his consciousness, all the Gods and the Nandi Bhrungi were awake with diyas lighted. In memory of this day, the devotees celebrate the Mahashivratri. On Krishnachaturdashi of Phalguna month, Mahadev evolved as a large piece of stone called Shiba Linga and thus this day is celebrated as Mahashivratri.
Importance:
It is believed that the tales of Mahashivratri have come from the great Epic Mahabharata. In the book, when Bhishma Pitamaha was lying down on the bed of arrows and advising on religion as what is right and what is wrong, he had mentioned the King of the Ikhyaku dynasty, Chitrabhanu. The wife of King Chitrabhanu celebrated Shivratri by keeping a fast. During this time, a sage visited the court of King Chitrabhanu and asked him the relevance of celebrating this festival. The king to explain the story to the sage began saying," O great saint, your request will be fulfilled. I am fortunate that I would be explaining the importance of this festival to you. In my previous birth, I was a hunter named Shumbara. One day we were looking for prey in the forest when it turned evening. Unable to return home, we took shelter in a wood apple tree to save ourselves from the wild animals. The whole night, we spent plucking the leaves of the tree and throwing them down. Tears also flowed down in memories of my family. Unintentionally, the leaves and my years fell on the Shiba Linga that was placed beneath the tree. When it was morning, we returned home with our prey. The next day I offered food to a beggar and then had my meal. After a few years when we died, we were lucky to get into Shaibaloka. Unknowingly, we fasted on Shivaratri, offered leaves and tears, due to which we got the blessings of the Lord. We also got to live in the Kailash Mountain. With His blessings, I became Chitrabhanu in the next birth and became the King."
In this manner, whosoever celebrates Shivaratri with all devotion and love, is blessed with freedom from illness and sadness. His spirit also gets to live in the Shaibaloka after death.
Celebration rules:
The devotees who celebrate Mahashivaratri, fast the entire day. In the evening, they visit the Shiv temple and light an Akhand Diya to stay awake all night. During the 4th prahar of night, the devotees chant the name of the Lord, and offerings are made. The festival is celebrated in four units of time. Every unit is for three hours. In the first unit, the Shiva Linga is coronated and bathed in milk. In the second unit, the curd is used for the coronation. In the third unit, honey is used. In the last unit, the mahadipa is taken to the top of the temple. This marks the end of the celebration. The major intention of this mahadipa is to remove the darkness of innocence from the entire world and fill the world with the light of knowledge. The Linga is worshipped with flowers, leaves, and fruits on every unit. Only on this day, the Lord is offered the Ketaki flowers. Since the devotees fast on this day while lighting the diyas and remain awake, this festival is also known as Jagara Osa or Jagara Brata. For self-welfare, there is a need for two things, infinite alertness and light of knowledge, which this festival teaches us.