Nishita Kala is defined as the 8th muhurta of the night and it is attained after doing complex calculation. It is of great importance to the Panchang makers and Jyothishis as this period is essential for calculating festival and auspicious days.
Nishita Kala period is chosen to do pujas and worship on the Shivaratri day. This period is of great importance to some Hindu communities. For a festival to be fixed there should be a Nishita Kala on the day.
For example a festival falling on Ashtami, there should be a nishita Kala on Ashtami day.
The Nishita Kala period is usually of duration of less than an hour and fall during the midnight.