From the Maha Shivaratri Wikipedia page:
Maha Shivaratri is a Hindu festival celebrated annually in reverence of the God Shiva.
Shivaratri is celebrated during the night by keeping a "jaagaran" - a night-long vigil with worship, unlike most Hindu festivals which are celebrated during the day. It is believed that Lord Shiva saved the universe from darkness and ignorance. This is the day when jyotiswaroop (luminous) shiva linga emerged between Brahma and Vishnu to pacify them. This day is also believed to be the day that Shiva was married to the Goddess Parvati. The Maha Shivaratri festival, also popularly known as 'Shivaratri' (spelt as Sivaratri, Shivaratri, Sivarathri, and Shivarathri) or 'Great Night of Shiva', marks the convergence of Shiva and Shakti. Maha Shivaratri is celebrated on the Krishna Paksha Chaturdashi of Bikram Sambat Calendar on the month of Falgun as per Amavasya-ant month calculation. As per Poornima-ant month calculation, the day is Krishna Paksha Trayodashi or Chaturdashi of Bikram Sambat calendar month Falgun which falls in February or March as per the English Gregorian calendar. Of the twelve Shivaratris in the year, the Maha Shivarathri is the most holy.
The festival is principally celebrated by offerings of Bael (Vilvam) leaves to Shiva, all-day fasting and an all-night-vigil (jagaran) with vedic, tantrik ritualistic worship of Shiva. All through the day, devotees chant "Om Namah Shivaya", the sacred mantra of Shiva. 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. On this day, the planetary positions in the Northern hemisphere act as potent catalysts to help a person raise his or her spiritual energy more easily. The benefits of powerful ancient Sanskrit mantras such as Maha Mrityunjaya Mantra increase greatly on this night.
In Nepal, Shivaratri is a national holiday. Millions of Hindus from around the world and Nepal itself attend Shivaratri together at the famous Pashupatinath Temple which is the holy Head of Lord Shiva. Thousands of devotees also attend Mahasivaratri at the famous Shiva Shakti Peetham of Nepal. Nepal Army takes a parade around the city to pay tribute to Lord Shiva and the holy rituals are performed all over the nations.
In Indo-Caribbean communities throughout the West Indies, thousands of Hindus spend the auspicious night in over four hundred temples across the country, offering special jhalls to Lord Shiva. In Mauritius, Hindus go on pilgrimage to Ganga Talao, a crater-lake, turned into the main Hindu prayer site of this sole Hindu majority African country. Maha Shivaratri is the main Hindu festival among Hindu diasporas from Nepal , Southern Indian States "Tamil Nadu, Kerala, Karnataka, Andhra Pradesh, Telengana" and Northern Indian State of Bihar.
On Maha Shivaratri, Nishita Kala is the ideal time to observe Shiva Pooja. Nishita Kala celebrates when Lord Shiva appeared on the Earth in the form of Linga. On this day, in all temples of Shiva, the most auspicious Lingodbhava Puja is performed.