Significance of महा शिव रात्रि (mahā śiva rātri - Great Night of Shiva)

Sacred மகா சிவராத்திரி (magaa sivaraaththiri)  today. Every month, the  night falling on the कृष्ण पक्ष चतुर्दशि (kṛṣṇa pakṣa caturdaśi -  fourteenth day of dark fortnight) that is, on the eve of அமவாசை திதி (amaavaasai  thithi - new moon day) is considered as மாத சிவராத்திரி (maadha sivaraaththiri - monthly shiva night) and is believed to be vibrant with spiritual energy. and hence the devotees observe a day-long उपवासम् (upavāsam- fasting) and keep vigil all night by indulging in prayers, meditation and other spiritual practices. In esoteric sense, the astronomical progress of the चन्द्र (chandra - moon) through चतुर्दशि तिथि (caturdaśi tithi - fourteen days) corresponds to the progress of the जीवात्म चैतन्य  (jīvātma caitanya – embodied soul’s consciousness) across the चतुर् दश मन्वन्तर (catur daśa manvantara– fourteen epochs of Manu)  viz. स्वयंभुव मनु (svayaṁbhuva manu), स्वरोचिश मनु (svarociśa manu)…देवसवर्नि मनु (devasavarni manu) & इन्द्रसवर्नि मनु (indrasavarni manu);  and the चतुर् दश रात्रि (catur daśa rātri – fourteenth night) represents the obscurative (dark) period at the end of that कल्प (kalpa - aeon) and the following அமவாசை திதி (amaavaasai  thithi - new moon day) represents the final dissolution of the individualized consciousness symbolized by चन्द्र (chandra - moon)  and its merger into the Supreme Divine Consciousness symbolized by the सूर्य  (sūrya- sun). The former is also theologized (in terms of bridal mysticism) as the sacred wedlock between नायकी / पार्वती (nāyakī  / pārvatī) who is wedlocked with नायक / परमशिव (nāyaka  / paramaśiva).          
In a  year, amongst all the மகா சிவராத்திரி (maasa sivaraaththiri - monthly shiva night), the one falling during the month of மாசி (maasi - mid feb-march) is considered the most sacred and hence referred as महा शिव रात्रि  (mahā śiva rātri - Great Night of Shiva). The importance of observing this sacred event is glorified in the शिव महापुराण (śiva mahāpurāṇa)  and various other scriptures 

शिवरात्रावाहोरात्रं निराहारो जीतेन्द्रियः।

अर्चयेदा यथान्यायं यथाबलमवञ्चकः॥

तत्फलं मम पुजायां वर्षमेकं निरन्तरम्।

तत्फलं लभते सद्यः शिवरात्रौमदर्चनात्॥

(

śivarātrāvāhorātraṁ nirāhāro jītendriyaḥ|

arcayedā yathānyāyaṁ yathābalamavañcakaḥ||

tatphalaṁ mama pujāyāṁ varṣamekaṁ nirantaram|

tatphalaṁ labhate sadyaḥ śivarātraumadarcanāt||

On the occasion of Shivaratri whosoever, controlling his sense organs, fasting totally for twenty-four hours, shall adore Shiva according to his capacity, without decietfulness, he shall earn the merit equal to performing of my worship for a year. The merit equal to my worship for a year shall also be achieved by a person who performs my adoration on the day of Sivaratri.

-translation by Shantilal Nagar

)- शिव महापुराण (śiva mahāpurāṇa) (2.9.12,13)

Mythologically, there are many  legends associated with this event as exemplified in the sacred scriptures. According to one legend, it is on this day, that  Lord परमशिव (paramaśiva) is believed to have manifested himself as the sacred महा लिङ्गोध्भव मूर्ति (mahā liṅgodhbhava mūrti) whose அடி (adi – feet/ bottom)  & முடி (mudi – head / top) could not be seen (measured) even by Gods viz. ब्रह्म (brahma) & विष्णु (viṣṇu) respectively. The शिव महापुराण (śiva mahāpurāṇa) very clearly narrates this incidence and summarizes that 

दिनमेतत्ततः पुण्यं भविष्यति महत्तरम्।

शिवरात्रिरिति ख्याता तिथिरेषा मम प्रिया॥

- शिव महापुराण (2.9.10)

(

dinametattataḥ puṇyaṁ bhaviṣyati mahattaram|

śivarātririti khyātā tithireṣā mama priyā||

Since then, the day is considered to be most auspiscious and yields merit. This is the date dear to Lord Shiva And is well known by the name of Shivaratiri

-translation by Shantilal Naga

)

Again, this is further testified by saint poet ஸ்ரீ கக்சியப்ப சிவாச்சாரியார் (shrI kachchiyappa sivaachchaariyaar) in the following verses from his magnum opus masterpiece கந்த புராணம் (kandha puraaNam)

அரியும் யானும்முன் தேடும்அவ் வனற்கிரி யனல

     கிரியெ னும்படி நின்றதால் அவ்வொளி கிளர்ந்த

          இரவ தேசிவ ராத்திரி யாயின திறைவற்

               பரவி யுய்ந்தனர் அன்னதோர் வைகலிற் பலரும்.

(

ariyum yaanum munthEdum avvanalgiri anala

girienumpadi ninRadhaal avvoLi kiLarndha

iravadhE sivaraaththiri yaayinadhu iRavaR

paraviyuyndhanar annathOr vagaliR palarum.

)

According to another legend, it is on this day, that  Lord परमशिव (paramaśiva) is believed to have married पार्वती ( pārvatī). According to Swami Sivananda, “This falls on the 13th (or 14th) day of the dark half of Phalgun (February-March). The name means 'the night of Shiva'. The ceremonies take place chiefly at night. This is a festival observed in honour of Lord Shiva. Shiva was married to Parvati on this day. People observe a strict fast on this day. Some devotees do not even take a drop of water. They keep vigil all night. The Shiva Lingam is worshipped throughout the night by washing it every three hours with milk, curd, honey, rose water, etc., whilst the chanting of the Mantra Om Namah Shivaya continues. Offerings of bael leaves are made to the Lingam. Bael leaves are very sacred as, it is said, Lakshmi resides in them. Hymns in praise of Lord Shiva, such as the Shiva Mahimna Stotra of Pushpadanta or Ravana's Shiva Tandava Stotra are sung with great fervour and devotion. People repeat the Panchakshara Mantra, Om Namah Shivaya. He who utters the Names of Shiva during Shivaratri, with perfect devotion and concentration, is freed from all sins. He reaches the abode of Shiva and lives there happily. He is liberated from the wheel of births and deaths. Many pilgrims flock to the places where there are Shiva temples”

Finally, it is on this day, that Lord परमशिव (paramaśiva) is supposed to have swallowed the हाल्हल विष (hālhala viṣa) that was emitted during the समुद्र मन्थन  (samudra manthana - churning of the ocean)

Related....

For details of my two cents on Nataraja Thathuvam...

https://whatisgod-chitsabesaayamangalam.blogspot.com/