Shiva Ratri

This year (2012), February 20th marks the anniversary of Lord Siva's appearance day, celebrating his advent from between Lord Brahma's eyebrows. Srimad-Bhagavatam, 12.13.16 says this (among other things) about Lord Siva:
vaishnavanam yatha shambhu, “Amongst all Vaishnavas, Lord Siva is the best."

Srila Prabhupada, in Krishna, the Supreme Personality of Godhead describes one observance of Siva Ratri by some of Krishna's greatest devotees:

"Once upon a time, the cowherd men of Vrindavana, headed by Nanda Maharaja, desired to go to Ambikavana to observe the Siva-ratri ceremony. The rasa-lila was performed during the autumn, and after that the next big ceremony is Holi, or the Dolayatra ceremony. Between the Dolayatra ceremony and the rasa-lila ceremony there is an important ceremony called Siva-ratri, which is especially observed by the Shaivites, or devotees of Lord Siva.

Sometimes the Vaishavas also observe this ceremony because they accept Lord Siva as the foremost Vaishnava. But the function of Siva-ratri is not observed very regularly by the bhaktas, or devotees of Krishna. Under the circumstances, Srimad-Bhagavatam states that Nanda Maharaja and the other cowherd men “once upon a time desired.” This means that they were not regularly observing the Siva-ratri function but that once upon a time they wanted to go to Ambikavana out of curiosity."

More:

• Video: Lord Shiva, with Chaturatma dasa
• Appearance of Lord Siva from Srimad-Bhagavatam
• FAQ: I thought the universe had many gods. What about the "Hindu trinity" - Brahma, Vishnu, and Shiva?"
• FAQ: Who is the Supreme Being - Lord Krishna or Lord Shiva?