Lord Shiva is worshipped in the form of a Jyothirlingam or "Lingam of light’’. Lingam representing His manifestation as a Creator of Good. Shiva means auspicious and linga means a sign or a symbol. Hence the shivalinga is regarded as a "symbol of the great God of the universe who is all-auspiciousness."
It is also a symbolic representation of Self (Atma linga) or of everything. Praying Shivalinga is considered as praying the Thrimurti in Absolute Form. In some Hindu denominations Brahma, Vishnu and Shiva represent the three primary aspects of the divine, and, are collectively known as the Trimurti - Brahma the creator, Vishnu the preserver, and Shiva the destroyer or transformer.
Lingas are depicted in various forms. One common form is the shape of a vertical rounded column.
Maha Shivarathri or Shivaratri is a festival celebrated every year on the 13th night/14th day in the Krishna Paksha of the month of Maagha or Phalguna in the Hindu Calendar (that is, the night before and day of the new moon).
The festival is principally celebrated by offerings of Bilwa (Bael) leaves to Lord Shiva, all day fasting and an all nightlong vigil.
Shiva is said to be an AbhishekaPriya . Rudrabhishekam is usually performed to a Lingam representing His manifestation as a Creator of Good.
Pradosham, the thirteenth evening twilight after the new moon, is considered is a very holy and auspicious day for the worship of Lord Shiva. The transition between sunrise to sunset on this day, every month is worshipped as Pradosha Kaala Pooja.