Maha Shivratri (the 'Great Night of Shiva') is a Hindu festival celebrated every year in reverence of Lord Shiva. Maha Shivratri is associated with Marriage of Shiva and Shakti.
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. On this day, the planetary positions in the Northern hemisphere act as potent catalysts to help a person raise his / her spiritual energy easily. The benefits of powerful ancient Sanskrit Mantras such as Maha Mrityunjaya Mantra महामृत्युंजय मंत्र increase manifold on this night.
After earth's creation was complete, Parvati asked Lord Shiva which devotees and rituals pleased him the most. The Lord replied that the 14th night of the new moon, in the dark fortnight during the month of Phagun, is his most favorite day. Parvati repeated these words to her friends, from whom the word spread over all creation.
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.