Maha Shivaratri is a festival that is celebrated in honor of Lord Shiva, the most important god in the Hindu religion.
This particular festival devoted to Shiva called Maha Shivaratri occurs once a year, however there are also festivals dedicated to this god in every single lunar-solar month of the Hindu calendar. The festival is observed principally by means of fasting and by means of worshiping the Lingam.
The festival known as Maha Shivaratri involves several foci, which are both spiritual and physical observances. Yoga and self reflection are central to this festival time. Lord Shiva and his female consort Shivatri are central to the Hindu religion.
The key story behind Maha Shivaratri is the tale of Lord Shiva drinking a deadly poison that would have destroyed all of the world. Shiva did not allow the poison to reach his stomach, where the universe was contained (or, rather, his stomach represents the universe). Instead, he trapped the poison in his neck, causing his neck to turn blue.