The 14th day of the dark half of every month- Krsna Chaturdasi- is called Sivarathri. The one in the month of Magha (February-March) is known as Mahasivarathri, since it is the greatest and darkest night of all.
Of all the major Hindu festivals, Mahasivarathri is the only one wherein the seriousness part is predominant. There is practically no festivity, revelry or gaiety in its observance, the whole thing being one of continuous solemnity. This is but natural since Siva is the god of the ascetics, the very incarnation of vairagya or renunciation. The basic disciplines to be kept up on this day are ahimsa (non-injury), satya (speaking the truth), brahmacharya (continence), daya (compassion), ksama (forgiveness) and anasuyata (absence of jealousy).
Fasting is one of the most essential aspects on this day. So is jagarana or keeping vigil in the night. Worship of Siva throughout the night, bathing the Sivalinga with panchamrta (five tasty things- milk, curds, ghee, sugar and honey), homa , japa (chanting) of the mulamantra (basic mantra) and prayer for forgiveness- are involved in its observance.