The great night of Lord Shiva. Celebrated because he saved the world by drinking poison that emerged during 'samudra manthan' (churning of ocean).
He held poison onto his throat as it was capable of destroying universe which is signified by Shiva's stomach. Hence the name "Neelkanth".
Devotees fast, perform yoga, meditation during the day & jaagran (staying awake) at night. 'Bel' leaves are offered to Linga. Water, milk, cow ghee & honey is also offered to Linga. Year has 12 Shivratris, this being the main.