On the day of Mahashivratri, Lord Shiva appeared as a Linga. Other beliefs are that Shiva got married today and drank the poison.
Dear children, I am delighted at your worship on this holy day. Henceforth this day will be famous as “Śivarātri” the holiest of holy days pleasing to me. He who performs the worship of my Liṅga and the embodied image on this day will be competent, to perform the task of creation and the maintenance etc. of the universe. The devotee shall observe fast on Śivarātrī, both during the day and the night. He shall perfectly restrain his sense-organs. He shall adore (with flowers) to the extent of his strength. He shall not deceive any one. By the worship on Śivarātrī day the devotee attains that fruit which usually accrues to one who continuously worships me for a year.
- Shiva Purana, Vidhyeshwar Samhita, Chapter 9.