Shivaratri is great festival of convergence of Shiva and Shakti. Each month, Chaturdashi Tithi during Krishna Paksha is known as Masik Shivaratri.
Masik Shivaratri in month of Magha is known as Maha Shivaratri according to Amavasyant School. However according to Purnimant School Masik Shivaratri in month of Phalguna is known as Maha Shivaratri. In both schools it is naming convention of lunar month which differs. However both, Purnimant and Amavasyant Schools, celebrate all Shivaratris including Maha Shivaratri on same day.
According to Indian Mythology, in the midnight of Maha Shivaratri, Lord Shiva was appeared in form of Linga. Shiva Linga was first worshipped by Lord Vishnu and Lord Brahma. Hence Maha Shivaratri is known as birthday of Lord Shiva and devotees worship Shiva Linga during Shivaratri. Shivaratri Vrat is popular since ancient time. In Hindu Puranas we get references of Shivaratri Vrat. According to scriptures even Goddess Lakshmi, Indrani, Saraswati, Gayatri, Savitri, Sita, Parvati, Rati observed Shivaratri fast.
11 January (Monday) Masik Shivaratri
10 February (Wednesday) Masik Shivaratri
11 March (Thursday) Maha Shivaratri
10 April (Saturday) Masik Shivaratri
09 May (Sunday) Masik Shivaratri
08 June (Tuesday) Masik Shivaratri
08 July (Thursday) Masik Shivaratri
06 August (Friday) Masik Shivaratri
05 September (Sunday) Masik Shivaratri
04 October (Monday) Masik Shivaratri
03 November (Wednesday) Masik Shivaratri
02 December (Thursday) Masik Shivaratri