Mahashivratri

Mahashivratri, perhaps the most significant Hindu festival, is celebrated on chaturdashi tithi of krishnpaksha of phagun maas (14th day of the lunar cycle).
Perhaps this explains the waning crescent on Lord Shiva's head.

Lord Shiva, the most compassionate God/Diety/Adi guru as well as the God of destruction, is also shown to have his third eye (pineal gland/seat of spirituality) open.

Ideally each 14th day of the waning moon phase is Shivratri, Mahashivratri being the most significant.

To everyone, this day holds its own significance. Some celebrate it as the union of Shiva and Shakti, his consort.
Some, celebrate it as the day when Lord Shiva attained Mount Kailash like stillness (deepest state of meditation).
I figure it means the same.
Vigyan Bhairav describes Shakti as the face of Shiva as well as the path unto Him. It also, in its various dialogues between Shiva and Shakti, describes the various paths to attain perfect state of mediation (or the state of Shiva) which also includes divine union.
So we can all agree that basically Shivratri is a night of spiritual evolution. It is celebrated by a day long fast and a night long vigil. A person who wants to observe this pious day stays upright all night long. The most significant time is that of midnight (calculated as the time exactly midway between sun set and sunrise) (not 0000hrs!)

The question is why night? Why darkness? Why waning moon? Most festivals especially those that are supposed to bring in prosperity, are observed during the waxing phase of the moon. Waxing phase is the phase that is observed by religions that follow the moon (which includes pagan/wiccan religions too) as the phase of 'increment'.
So why the waning phase for mahashivratri?
Dark is sadly associated with the devil, by the naive. Dark (and waning moon) signifies 'banishing'. Each month, Shivratri encourages us to banish ignorance and attain spiritual advancement, Mahashivratri being the most powerful night amongst them all. Similar banishing rituals are held by those who follow Paganism/Wicca.

Shiva, the destroyer, destroys 'all that is' like material objects around us so we may attain 'all that isn't' (Shiva) that is void/dark depths and truths of the universe.

Light creates distractions by showing us all sorts of illusions. When there is no light, there are no illusions, only truth.

Truth alone can help us all evolve as a species. In the absolute truth, there is love compassion and kindness. Thus, Lord Shiva is known as the most compassionate-The destroyer of 'all that is' and the giver of knowledge and truth...

Mahashivratri is an opportunity to seek and ask, an opportunity to grow and evolve.

Happy Mahashivratri!
Namaste