Pradosh Vrat
Pradosh is an auspicious day dedicated to Lord Shiv on the 13th day of a lunar fortnight in Hindu traditional calendar. Pradosh occurs during the two different phases of Moon (waxing and waning) in a Hindu Lunar month. Prayers and Poojaa are offered to Lord Shiv and Goddess Paarvatee during the evening twilight period on this day. When Pradosh falls on a Tuesday, it is known as Bhaum Pradosh. Pradosh as the name indicates is a period 1 and 1/2 hours before the sunset and 1 and 1/2 hours after the sunset. Fasting starts from sunrise and ends with the evening Poojaa. Most devotees prefer to pray to Lord Shiv or remain in Shiv temples during the Pradosh period.

Types of Pradosh
There are five types of Pradosh --

(1) Nitya Pradosh -
The evening time on all days, between just 3 Ghatee (72 minutes) before sunset and the time when the stars rise or become visible in the sky. This is Nitya Pradosh and may be observed daily.
(2) Paksh Pradosh -
Sandhyaa (evening) time of Shukla Paksh Chaturthee (4th Lunar day after New Moon) every month.
(3) Maas Pradosh -
Sandhyaa time on Krishn Paksh Trayodashee (13th Lunar day after Full Moon) every month.
(4) Shani Pradosh or Mahaa Pradosh  -
Sandhyaa time of Krishn Paksh Trayodasheei which falls on a Saturday is called Shani Pradosh. Take bath one hour before, do Poojaa of all the family of Shiv - Ganesh, Paarvatee, nandee and Kaarttikeya. Then invoke Shiv by chanting "Om Namah Shivaaya" in a special Kalash (Pitcher), placed on a square Mandal, drawn a lotus in it, and pread with Darbhaa grass. After formal worship is completed, Shiv Raatri story or Shiv Puraan is read. One may recite Mahaa Mrityunjaya Mantra also 108 times. In the end the sacred Kalash water is taken, the sacred ash is applied to the forehead. A gift of a pot, a cloth and an image of god have to be given to a Braahman or in charity to conclude the worship. Even a single Bilva leaf (leaf taken from the wood-apple tree) offered to the Lord at this unique auspicious moment equals a 100’s of Mahaa Poojaa.
(5) Pralaya Pradosh -
The time when the entire universe gets annihilated and is merged with Shiv. This Pradosh Vrat is observed on every 13th Lunar day after full and new Moon, by the wife and husband jointly with the hope of being free from, miseries or for gaining material prosperity. They should bath early in the morning, adore Shiv, and fast during the day, and after bath in the evening, perform Poojaa to Lord Shiv and his consort Paarvatee with the offerings of flowers and Naivedya. They then take their supper or meal. This Vrat is brought to a close at the end of a year.
(6) Som Pradosh or Saumya Pradosh -
When Pradosh falls on a Som Vaar (Monday), it is called Som Pradosh.
(7). Bhaum Pradosh -
When a Pradosh falls on a Mangal Vaar (Tuesday), it is called Bhaum Pradosh.

How to Do This Vrat
The Skand Puraan relates how Shaandilya Muni prescribed this Vrat to a certain Braahman woman.

There are two ways to do this Vrat.
--Some people observe a 24-hour fasting, which includes not sleeping at night.
--Another method is fasting from sunrise to sunset and after Shiv Poojaa the fast is broken.
Since Pradosh is a Kaal - period (from 1.30 hours before sunset to 1.00 hour after sunset) all Poojaa is done during this period only. If there is a temple of Shiv, one can go there too to perform Poojaa.
--Some devotees do not fast but worship Shiv during the Pradosh period or visit temples.

Take the bath an hour before sunset, and first perform a preliminary worship of Shiv, together with all the others of his Divine family, namely, Nandee, Shiv, Paarvatee, Ganesh, and Skand - in this order. After the worship of Ganesh, Shiv is invoked in a special Kalash placed on a square Mandal with a lotus drawn on the Kalash and Darbhaa grass spread over the Mandal. After the formal worship has been completed, a Pradosh story is read and heard by the devotees. This is followed by the recitation of the Mahaa Mrityunjaya Mantra 108 times. In the end the sacred Kalash water is partaken of, the sacred ash is applied to the forehead, and the water which was used to bathe the Lord, is drunk. A gift of a pot, a cloth and an image of the Lord is given to a Braahman to conclude the worship. Lighting a single lamp at this time is very meritorious.

During Prodosh time anointing (Abhishek) the Shiv with the following is considered fruitful. "Oh Rudra, My salutations to your anger and to your arrows. My salutations to your bow and to your two hands." Rudra is always known to be easily pleased and through the praise contained in Shree Rudram, the fierce Rudra is calmed and becomes ready to grant us every conceivable blessing as requested in the Chamakam hymn. Bilva Patra (leaves) and flowers are most suitable leaves and flowers for Shiv Pooja. If situation permits, go to Shiv temple for every Pradosh and live in His grace blissfully.

Food to be Taken
Although one should not take salt in this Vrat. Staunch devotees opt for water only throughout fasting time and will only eat the Prasaad offered in the evening. Such devotees only eat cooked food from next day morning. But as people know, there is not much hard and fast rules in Hindu Vrat and festivals. Whatever is available according to time, place and one's physical condition may be used for worship and Vrat. One should never worry about all this. It is rather important to lift oneself spiritually by self control.

A very important point to be remembered in this connection is that during this auspicious period all the hosts of celestial beings and gods come down from the Heavens and attend the worship in their subtle forms. This adds immensely to the sanctity of the worship. This Vrat is highly lauded by the scriptures and is of very great sanctity and importance to worshippers of Shiv. The mere sight of the Deity in a temple during this period will destroy all sins and bestow bountiful blessings and Grace upon the fortunate beholder. Even a single Bel leaf (Bilva Patra) offered to the Lord at this unique, auspicious moment equals a hundred Mahaa Poojaa. It is usual to have special additional lights in the shrine during the Pradosh. To light even a single wick at this juncture is highly meritorious and productive of untold benefits, spiritual as well as material. Most fortunate and blessed is the person who performs the Pradosh Vrat, for upon him Shiv showers his choicest grace and blessings in a very short time. According to Ved, worship Lord Vishnu in the morning and Lord Shiv in the evening, especially during Pradosh Kaal is highly beneficial to the Saadhak, especially when the 12th Tithi (Dwaadashee) in Krishn Paksh or Shukla Paksh ends before midnight.

Pradosh Mahaatmya
[From Shiv Puraan]

Shiv Puraan gives its importance in the following 8 Shlok - "During the time of Pradosh, the Lord who holds the trident, makes Goddess Gauree who is the mother of all the three worlds, sit on a golden seat inlaid with precious gems and prepares to dance himself, and all the Dev sing his praise at this time, the goddess of Knowledge plays Veenaa, the hundred faced Indra plays the flute, the Brahmaa who was born in a lotus flower keeps time, the Goddess Lakshmee starts to sing, the God Vishnu plays the drum with ease, and all the Dev stand all round in service, and pray Lord Shiv.

When the time of Pradosh arrives, Gandharv, Yaksh, birds, snakes, saints, Vidyaadhar, Dev, the celestial dancers, Bhoot, and all the beings in the three worlds, come and stand near the Lord Shiv. So at the time of Pradosh, there is need to worship only Shiv, instead of Vishnu, Brahmaa and others, for the proper worship of Lord Shiv then, would give the effect of worshipping all gods. Let us be protected by that dancing Shiv, who uses the various sounds of the shaking drum, to call all those people drowned in the fearful life, who indicates protection to them using his right hand held down, who lifts his left leg capable of killing all sorrows, to those who fall at his feet, who indicates the path of salvation by his left hand held up, and who shows that all this is true by carrying fire in the other left hand.

I tell the truth, I tell what is good for the other world, I tell the views of the Upanishad, for every insignificant animal that is born, the only meaningful thing is the service to God. He who does not worship Lord Shiv during Pradosh, he who does not even bow before him at that time, he who does not at least listen to the story of Shiv at that time, is a foolish soul who would be always poor, birth after birth. He who during the time of Pradosh, worships Lord Shiv, with full concentration using lotus flowers, would forever along with his children, wife and friends, get all wealth and all luck in a very large measure."

Story Behind the Pradosh
Once Devtaa and Asur were trying to get Amrit (nectar) from the Khseer Saagar (milk ocean) using the serpent Vaasuki as a churning rope and Mandaraachal Parvat as a churning rod. Then the terrible poison Halaahal came up. All got frightened and pleaded to Lord Shiv for rescue. Being the peak of mercy Lord Shiv drank that poison. After that they resumed their effort to get Amrit. They got it on Dwaadashee (twelfth Moon day). Without praying and thanking the God, by whose grace they got this Amrit, Devtaa started dancing and celebrating their victory. On Trayodashee (13th Moon day) they realized that they had sinned of not praying Shiv and pleaded for his forgiveness. The pleased graceful Shiv forgave them and danced between the horns of the Nandee (Holy Bull). It was the time Pradosh time. So whoever prays Shiv during that time, Shiv fulfills their wishes and give them Mukti.