Most people must be aware of the Mahashivaratri but for hardcore Shiva devotees there is a Shivratri in each month. It happens on the ‘Krishna Paksha Chaturdasi’ of each lunar month. ‘Krishna Paksha’ is the dark fortnight of a lunar month.
Thus there is a Shivaratri in January, February, March, April, May, June, July, August, September, October, November and December. The Mahashivaratri occurs in February or March and it is also on ‘Krishna Paksha Chaturdasi.’
The Krishna Paksha Chaturdasi usually occurs the day before of Amavasi.