Shivaratri is celebrated on the 6th night of the dark Phalgun (Feb or March) every year. On the auspicious day, devotees observe fast and keep vigil all night. Mahashivaratri marks the night when Lord Shiva performed the ‘Tandava’. It is also believed that on this day Lord Shiva was married to Parvati Ma. On this day Shiva devotees observe fast and offer fruits, flowers and bel leaves on Shiva Linga.
Maha Shivratri or Maha Sivaratri or Shivaratri or Sivaratri (Night of Shiva or “Great Night of Shiva”) is a Hindu festival celebrated every year on the 13th night/14th day in the Krishna Paksha (waning moon) of the month of Maagha (as per Shalivahana) or Phalguna (as per Vikrama) in the Hindu Calendar (that is, the night before and day of the new moon). The festival is principally celebrated by offerings of Bael (Bilva) leaves to the Lord Shiva, all day fasting and an all night long vigil.
This blog site is very good.