The legend of Lord Shivas assurance explains the popularity of Shivaratri festival amongst the devotees. Besides, it also tells us why we observe fast in the name of the Lord and perform ceremonial baths on a Shivaratri day.
Story goes that after creation was complete, Lord Shiva began to live
on top of the mount Kailash with his consort Parvati. One day, Goddess
Parvati asked Shiva that his devotees perform many rituals to please the
Lord, but which one pleases him most. To this, Lord Shiva replied that
the 14th night of the new moon, during the month of Phalgun is his
favorite day. The day is celebrated as Shivaratri. On this day, devotees
observe strict spiritual discipline and worship Shiva in four different
forms during each of the four successive three-hour periods of the
night. Lord further said that the devotees offer him bel leaves on the
day and that those leaves were precious to him than the precious jewels
and flowers. Explaining the right way of worshipping to Shiva, the Lord
said, devotees should bathe me in the milk during the first period, in
curd at the second, in clarified butter in the third and in honey in the
fourth and the last period. Lord Shiva further added that next morning
devotees must feed the Brahmins first and, only after following the
prescribed ritual must he break the fast. Shiva further told Parvati
that rituals of Shivaratri could not be compared with any other.
Goddess Parvati became deeply impressed with Lord Shivas speech
and she narrated it to her friends. Through them the word spread all
over the creation. Hence, Shiva devotees began to celebrate Shivaratri
by fasting and by performing the ceremonial baths and making an offering
of bel leaves.