Unlike other festivals associated with Hinduism, Shivratri is a night dedicated largely to prayers and not to celebrations. Of course, when Lord Shiva is worshipped there is joy all around but that joy is of Brahman realization.
The importance of Shivaratri is closely associated with ‘amavasi’ (the no moon night or full dark night)). Amavasi symbolically represents Kaliyuga. Lord Shiva appeared just before the beginning of Kaliyuga to rid the world of evil and ignorance, which is symbolically represented through Amavasi. Therefore Mahashivratri is celebrated to get rid of evil and ignorance.
On the Shivratri day, Lord Shiva is worshipped in a special form of Linga called ‘Lingodabhavamurti.’ It is a lingam in the form of fire which has neither a beginning nor an end. It must be noted here that ‘linga’ means ‘sign.’ And it is merely an attempt to capture the formless or Brahman.
There are numerous legends and myths associated with Shivratri, which is mainly found the Puranas associated with Shiva. An important myth is that Shivaratri is the birthday of Lord Shiva – this is because the formless Lord Shiva appeared for the first time in the form of Lingodabhavamurti or Jyotirlinga before Lord Vishnu and Brahma.
Other important myths include that of Tandava, the consummation of poison during the churning of ocean (Samudramanthan), hunter accidentally dropping the leaves of bilva, the loss of importance of Ketki flower, which is now only offered during Shivratri.
Incidentally, there is a Shivratri in each month of the Hindu calendar. This is known as Masa Shivratri and is observed by staunch Shiva devotees.
Another important Shivratri in the year is during Shravan month (July - August). This is part of the Masa (or monthly) Shivratri observed by staunch Shiva devotees.