Maha Shiva Ratri occurs on the new moon--February 17th this year.
One interpretation of this festival is that it represents a mystical celebration of the marriage of the 'twin realities' of Shiva energy (male energy) with Shakti (female energy, the primordial force of femininity or Universal Mother)
Bol! Bam! O Namo Shivaya! O Namo Parvatiya!
(how about a Parvati Ratri festival, too?)