Maha Shivratri (the 'Great Night of Shiva') is a Hindu festival celebrated every year in reverence of Lord Shiva. Alternate common spellings include Sivaratri, Shivaratri, Sivarathri, and Shivarathri. It marks the convergence of Shiva and Shakti. Maha Shivratri is celebrated on the Krishna Paksha Chaturdashi of Hindu calendar month Maagha which falls in February or March as per the Gregorian calendar.
The festival is principally celebrated by offerings of Bael or golden apple or Bilva/Vilvam leaves to Lord Shiva, all-day fasting and an all-night-long vigil (jagarana). All through the day, devotees chant "Om Namah Shivaya", a sacred Panchakshara mantra dedicated to Lord Shiva. In accordance with scriptural and discipleship traditions, penances are performed in order to gain boons in the practice of Yoga and meditation, in order to reach life's highest good steadily and swiftly.
In Nepal, millions of Hindus attend Shivaratri together from different part of the world at the famous Pashupatinath Temple. Thousands of devotees also attend Mahasivaratri at the famous Shiva Shakti Peetham of Nepal.
On Mahashivratri, Nishita Kala is the ideal time to observe Shiva Pooja. Nishita Kala is the time when Lord Shiva appeared on the Earth in the form of Linga. On this day, in all Shiva temples, the most auspicious Lingodbhava puja is performed.
Here're three reasons to celebrate Shivratri:
1. The absolute formless God, Sadashiv appeared in the form of "Lingodbhav
Moorti" exactly at midnight on Maha Shivratri. That is why all Shiva devotees
keep vigil during the night of Shivratri and do "Shivlingam abhishekham" at midnight.
2. Lord Shiva was married to Devi Parvati on Shivratri. Remember Shiva minus
Parvati is pure 'Nirgun Brahman'. With his illusive power, (Maya, Parvati) He
becomes the "Sagun Brahman" for the purpose of the pious devotion
of his devotees.
3. It is also believed that on Shivratri, Lord Shiva became 'Neelkantham' or the blue-throated by
swallowing the deadly poison that came up during the churning
of "Kshir Sagar" or the milky ocean. The poison was so deadly that even a drop in His
stomach, which represents the universe, would have annihilated the entire world.
Hence, He held it in His neck, which turned blue due to the effect of poison.
Shivratri is therefore also a day of thanksgiving to the Lord for protecting
us from annihilation.