Maha Shivaratri is observed once a year in Phalgun month and marks Lord Shiva's appearance as a Shivling, making it the most important Shivratri. Shivratri, also called Masik Shivratri, is observed every month on Krishna Paksha Chaturdashi. Both involve night worship, but Maha Shivaratri holds greater spiritual and religious importance. Maha Shivaratri will be celebrated on Sunday, February 15.