Shivaratri is one of the most auspicious vrata-s for devotees of Lord Shiva. The puranas explain the grand significance of this vrata. The Skanda Mahapurana, particularly, gives details on the way to observe the vrata.

Maha Shivaratri - 4-Kaala Puja Timings

The four Kala Puja Timings are as below - 
First Kala Puja - 6:30 - 9.30 pm; 
Second Kala Puja - 9:30 - 12.30 pm; 
Third Kala Puja - 12:30 - 03.30 am; 
Forth Kala Puja - 3:30 - 6:30 am.

History of Shivaratri

Once when everything, in all the worlds was withdrawn into Lord Shiva and and in that darkness of dissolution, the Divine Mother Parvati worshipped Lord Shiva as laid down by the Agamas. Parameshwara, pleased, blessed Her with a boon. So that all creatures may benefit in future, She asked that going forward whoever worships the Lord on the day of Shivaratri with devotion, be blessed with ultimate liberation. Pashupati granted it thus paving the way for all of us to get blessed easily. 

When Brahma and Vishnu were engaged in battle between themselves as to who the greatest was, Lord Shiva appeared before them as an infinite column of fire. The day that the column of fire appeared is celebrated as – Tiru-Kartikai. Brahma and Mahavishnu were not able to find the start or end of that column. They repented their folly and prayed to Lord Shiva for forgiveness. The night when Lord Shiva appeared before them from the column of fire as a shiva-linga that they can worship and blessed them came to be known as Shivaratri. For this reason the midnight of Shivaratri is hailed as the Lingodbhava Kala. 

Devotees pray to the Lord throughout the night on this day by performing abhisheka, chanting His holy names and performing other sacred deeds. Every month the chaturdashi of the krishna-paksha (14th day of the dark fortnight of the lunar month) is called masa Shivaratri. The one that comes in the month of "Magha" (mid-February to mid-March) is called Maha Shivaratri. It is considered as the most important vrata by the devotees. There are many incidents narrated describing the greatness of this day.

Once a hunter, at nightfall, after a failed hunt, tired and hungry, was chased by a tiger. He climbed up a tree to escape the tiger. It so happened that it was a Bilva tree. The tiger sat under the tree waiting for him to come down. The hunter sitting on a branch of the tree knew that he could not fall asleep. He started plucking the leaves and dropping them down to pass his night. Below the tree there was a Shiva linga. The whole night was spent like this. God was pleased with the upavasa (abstinence from food) and the puja (worship) performed by both the hunter and the tiger unknowingly. Out of His infinite Grace, he bestowed the hunter and the tiger with moksha (final beatitude).

Once in a temple of Shiva, on an auspicious Maha Shivaratri, the lamp at the altar was very dim. At that time a mouse which came to catch its prey touched the flame. The heat caused it to move its head immediately. In the process it re-kindled the lamp and the altar was brightly illuminated again. Lord Shiva, pleased by this deed made the mouse Mahabali, the renowned asura king. There are many incidents like this narrated in our Puranas. All emphatically state that this vrata, performed with pure devotion and love, ensures the Grace of the Almighty.

Shivaratri Observance – When?

Skanda Purana describes four Shivaratri-s. The first one is nitya Shivaratri (daily observance of shivaratri - worshiping Lord Shiva at mid-night). The second one is the masa Shivaratri, observed every month on chaturdashi of the krishna-paksha (14th day of the dark fortnight of the lunar month). The third one is the Magha prathamadi Shivaratri which is observed for the thirteen days starting from prathama tithi (1st day of the waning phase) in the month of Magha (Masi) and culminating on the chaturdashi night when the Lord is worshiped throughout the night. The fourth one is observed in the month of Magha (Masi) on the night of chaturdashi of the krishna-paksha. The observance of this Shivaratri is most widespread and is called Maha Shivaratri.

Shivaratri Observance – How?

On the day of Maha Shivaratri, one wakes up early in the morning and meditates on the Infinitely Auspicious, adorned with thousands of splendid garlands, in the form of a maha-linga. Purifying the body with a bath and worshipping Lord Shiva in the form of shiva-linga is done at the four jamas of the night (four equal time intervals of the night). The worship of the shiva-linga is explicitly mentioned for this vrata as it is time the Lord out of His Grace, for the benefit of pashu-s appeared as a “formless-form” that is symbolised by the linga.

See Also: 
1. Shiva ratri puja - in Tamil; in Bengali ; in Hindi ; in Gujarati ; in Kannada ; in Malayalam; in Marathi ; in Telugu ; in Roman script 
2. Shivaratri dates for this year 
3. Thirukkarthikai