Shivarathri is one of the main festivals of Lord Shiva in the form of Shiva lingam a form that Lord Brahma and Vishnu couldn't find the head and toes of Lord Shiva. There are different Shiva rathris, Nithya Shivarathiri (Daily), Monthly Shivarathiri, the one that falls in the tamil month of Maasi is Maha Shivarathiri