JAMRAT E WUSTA
Jamrat is a Hajj ritual that involves stoning the three pillars that represent the devil (shaytan in Arabic). As part of this Hajj ritual, the pilgrims throw small pebbles at those three structures of Jamrat in the city of Mina. Pilgrims collect these pebbles at Muzadlifah. These pebbles (small stones) can also be collected in the city of Mina, east of Makkah. The process of casting or stoning of seven pebbles to these pillars is called as Rami or Ramee.
The Middle pillar which is closed from Al-Khaif Mosque, is known as the pillar of wusta (Jamra e Wusta). And this the pillar which is stoned on 11th 12th and 13th of Zilhijjah along with other 2 pillars (Jamra E Uqbah and Jamra e Ulaa) during Hajj days.
Stoning the Jamaraat is one of the obligatory rituals of Hajj and is prescribed for everyone who intends to do this great pilgrimage. This ritual is mentioned clearly in the Sunnah and its authenticity is agreed upon by the scholars.
The historical significance of Jamrat ritual hails from Prophet Ibraheem when the devil appeared to him three times. At each of the appearances, Angel Jibraeel (Gabriel) told Prophet Ibraheem (Abraham) to pelt the devil. Thus, the basis of the significance of the ritual.  
The first (minor) pillar called as Jamra e Ulaa (Sughra) is close to the Al-Khaif Mosque, 155 meters from the first pillar is the second (medium) pillar (Jamra e Wusta). The third (major) pillar, another 155 meters on, is known as the pillar of Aqabah (Jamra e Uqbah or Kubra). Pilgrims start the stoning with the minor pillar and then move on to the medium pillar and the major pillar. 
Narrated 'Aishah(R): The Messenger of Allah (ﷺ) performed the obligatory circumambulation of the Ka'bah at the end of the day of sacrifice after he had offered the noon prayer. He then returned to Mina and stayed there during the tashriq days and he threw pebbles at the jamrahs when the sun declined. He threw seven pebbles at each of the jamrahs, uttering the takbir (Allah is most great) at the time of the throwing the pebble. He stood at the first and the second jamrah, and prolonged his standing there, making supplications with humilation. He threw pebbles at the third jamrah but did not stand there. Sunan Abi Dawud 1973 
With regard to stoning the Jamaraat on the days of al-Tashreeq – the 11th, 12th and 13th of Dhu’l-Hijjah – that starts after the sun has passed its zenith – i.e., midday when the time for Zuhr begins – and lasts until nighttime. If it is too difficult because of overcrowding etc, there is nothing wrong with stoning the Jamaraat at night, until dawn. It is not permissible to stone the Jamaraat on the 11th, 12th and 13th before midday, because the Prophet (peace and blessings of Allaah be upon him) did not stone them until after midday, and he said to the people: “Learn from me your rituals (of Hajj).” Sahih Muslim 1297 and Sunan an-Nasa'i 3062.
The fact that the Prophet (peace and blessings of Allaah be upon him) delayed stoning the Jamaraat until this time, even though it was very hot, and did not do it earlier in the day, when it is cooler and easier, indicates that it is not permissible to stone the Jamaraat before this time. This is also indicated by the fact that the Prophet (peace and blessings of Allaah be upon him) used to stone them from the time when the sun passed its zenith before he prayed Zuhr. This indicates that it is not permissible to stone the Jamaraat before the sun passes its zenith, otherwise doing that would be better so that one could pray Zuhr when its time began, because it is better to pray when the time for that prayer begins. The point is that the evidence indicates that stoning the Jamaraat on the days of al-Tashreeq is not permissible before the sun has passed its zenith. Fataawa Arkaan al-Islam, p. 560
The days of al-Tashreeq end at sunset on the 13th. However stoning during the day is better unless – because of the large numbers of pilgrims and their thoughtless attitude towards one another – one fears death, harm or unbearable hardship, in which case it is permissible to stone the Jamaraat at night and there is nothing wrong with that. If a person stones them at night even though he does not fear these things, there is also no harm in that, but it is better to be on the safe side in this matter and not stone them at night unless there is a need to do so. 
Ibn al-Mundhir (may Allaah be pleased with him) said: They were unanimously agreed that if a person stones the Jamaraat during the days of al-Tashreeq after the sun has passed the meridian, that is valid. AL-IJMAA’ BY IBN AL-MUNDHIR (11)
Ibn Hazm (may Allaah be pleased with him) said: They agreed that the three days after the Day of Sacrifice are the days for stoning the Jamaraat, and that the one who stoned them during those days after the meridian, that is valid. MARAATIB AL-IJMAA’ BY IBN HAZM (46)
Ibn Qudaamah (may Allaah have mercy on him) said: When he reaches Mina, he should start with Jamrat al-‘Aqabah, which is the furthest of the Jamaraat from Mina and the closest of them to Makkah, and it is at al-‘Aqabah, hence it is also called Jamrat al-‘Aqabah. He should stone it with seven pebbles, saying takbeer with each throw. He should stand at the middle of the valley and turn to face the qiblah, then he should leave and should not stay there. This is a summary of the views of those scholars whose views we know. Al-Mughni (3/218) 
There are many mistakes committed by some pilgrims when stoning the Jamaraat.
  http://islamqa.info/en/125711