Night usually represents evil then why is Shivaratri celebrated during night? The night after Mahashivratri is ‘amavasya’ (full dark night). A day when the world will be completely dark. Symbolically, nothing but only ignorance and injustice will prevail. This ‘amavasya’ also represents ‘Kaliyuga.’ Lord Shiva appeared just before the beginning of Kaliyuga to rid the world of ignorance and evil. This was during the night before ‘amavasya.’
Therefore special worship is done before ‘amavasya’ to please Lord Shiva who is the remover of darkness, evil and ignorance.
Then, almost all the myths and legends associated with Shivaratri happened during night and this is another reason.