Shivaratri is a period of special spiritual significance and observance sacred to the Hindu god Shiva.
Its timing is determined by the coincidence of Chaturdashi tithi during Krishna paksha with the nightly period of Nishita-kaal. This typically occurs once in each lunar month and is observed by some as Masik Shivaratri (or Masa Shivaratri). The monthly observance of Shivaratri, indefinitely or for a year in place of a 12-year annual observance, is described to King Bhoja by sage Sārasvata in the Skanda Purana [1].
Shivaratri is more often observed annually as the festival of Maha Shivaratri, one of the most significant religious observances of the Hindu year and the most significant for followers of Shiva. Maha Shivaratri is observed when Shivaratri falls within the Amanta lunar month of Magha, or the Purnimanta lunar month of Phalguna.
Sawan Shivaratri is observed when Shivaratri falls within the lunar month of Sawan, also known as Shravana, which is sacred to Shiva. Sawan Shivaratri is more popular in northern India where the Purnimanta lunar calendar is used, so its date is set by the coincidence of Shivaratri with the Purnimanta lunar month of Shravana, during the Amanta lunar month of Ashadha.
Ardent devotees of Shiva observe Nitya Shivaratri during Nishita-kaal every night.