Mahdi (as) is a messianic figure in Islamic eschatology who is believed to appear at the end of times to rid the world of evil and injustice. He is said to be a descendant of Prophet Muhammad (PBUH& HP) who will appear while Jesus Christ (Prophet Isa-ibni Maryam) accompanying him and will lead the believers to rule the world and fill it with justice and peace. This is what the Quran says:

 And We wanted to confer favour upon those who were oppressed in the land and make them leaders and make them inheritors. (Al-Qasas 28:5)

Who is Mahdi (as)? 

The name Mahdi is derived from the Arabic root h-d-y, commonly used to mean "divine guidance". Though Imam Mahdi features in both Shia and Sunni understandings of Islam, they differ in his attributes and status. Among Twelver Shi’as, Mahdi is believed to be the son of the eleventh Imam, Imam Hasan al-Askari (d. 874), and is said to have been on divine occultation (ghaybat) by God’s will. However, the Sunnis assert that he (Imam Mahdi) is a descendant of Lady Fatimah (SA), the daughter of the Prophet Muhammad, and it is unknown whether he was born or not.

What is the View of Muslims About Mahdi (as)?

According to our belief, one of the most extraordinary acts of worship is to wait for the promised saviour, and this waiting means trying to provide the conditions for his reappearance.

In the Shi'a Islam, or the school of Ahlul-Bayt, the eschatological Mahdi was commonly given the epithet al-Qa'im, which can be translated as “he who will rise”, signifying his rise against tyranny in the end of all times. Distinctively Shi'a is the notion of the temporary occultation of Imam Mahdi, whose life has been prolonged by God’s will.

As in above mentioned, the Sunnis also believe in the promised Mahdi and many hadiths about him have been narrated in their books. Abu Dawood quotes Prophet Muhammad (PBUH & HP) as saying: " Mahdi will be from my household, from the descendants of Fatimah”. And another hadith states: “Even if only one day remains [until the doomsday], God will lengthen this day until He calls forth a man from me, or from the family of my house, his name matching mine and his father's name matching that of my father. He will fill the Earth with equity and justice just as it had previously been filled with injustice and oppression”. (Furnish, 2005, p. 14) According to narrations before the arrival of Imam Mahdi, the earth would be filled with chaos. While divisions and civil wars, moral degradation, and worldliness would be prevalent among people, injustice and oppression would be rampant throughout the world. The Dajjal (The Anti-Christ) would appear and will spread decadence and corruption in the world. With an army bearing black banners, which would come to his aid from the east, Imam Mahdi would confront the Dajjal. Jesus would pray behind Imam Mahdi and then will kill the Dajjal. The Sufyani, another representative of the forces of the dark, also features in the traditions. He will rise in Syria before the appearance of Imam Mahdi. When the latter appears, the Sufyani, along with his army, will either be swallowed up en-route to Mecca by the earth by God's command or defeated by Imam Mahdi.

What Role does he have in the Future of Humanity

In Imamiyyah (Twelver Shiism), the largest Shi’a branch, believing in the messianic imam is not merely a part of the creed, but is the pivot. For the Twelver Shi'a, Imam Mahdi was born but disappeared after a while, and he would remain hidden from humanity until he reappears to bring justice to the world at the end of time, a doctrine known as the Occultation. This Imam on occultation is the twelfth imam, named Muhammad, son of the eleventh Imam, Hasan al-Askari. According to the Twelvers, the Mahdi was born in Sammara around 868, though his birth was kept hidden from the public. He lived under his father's care until 874 when the latter was killed by the Abbasids. 

Shi'a scholars have argued that the prolonged lifespan of Imam Mahdi Mahdi is not at all unreasonable given the longevity of  the Prophet Noah, Khidr and Jesus, as well as secular reports about long-lived people. According to scholars Imam Mahdi is viewed as the restorer of true Islam, and the restorer of other monotheistic religions after their distortion and abandonment. He establishes the kingdom of God on earth and Islamizes the whole world. As a matter of fact, in their true form, it is believed, all monotheistic religions are essentially identical to Islam as "submission to God." It is in this sense, that one should understand the allegation that al-Mahdi will impose Islam on everyone. His rule will be paradise on earth, which will last for several years until his decease.[1]

References:

1. (Sachedina, 1981, p. 176-178)

Eid al Adha (or Dhuha) is one of the greatest eids of Islam. It is a feast of love, sacrifice, and submission to the will of Allah (s.w.t). Celebrated on the 10th of Dhul Hijjah, the last month of the Islamic calendar, it is the time of the year where Muslims from all over the world gather for the annual ritual of Hajj and mark the end of Hajj by sacrificing a camel, a cow or a sheep. Those Muslims who could not make it for Hajj, sacrifice these animals and distribute the meat and other gifts among the poor, their family and friends. 

There are many recommended (not obligatory) rituals and practices to be performed on the day of Eid al Adha, some of them are as follows: 

-Doing Ghusl (ritual bath)
-Eid Prayer, either in a congregation or individually
-Reciting the Ziarat of Imam Husain (AS), the third infallible Imam
-Sacrificing either a camel, cow, or sheep. Sacrifice is obligatory for those performing Hajj and recommended for those not performing Hajj
-According to Islamic laws, it is forbidden to fast on this day

History of Eid al Adha 

Prophet Abraham (PBUH) was a true embodiment of the verse from the Holy Quran: “Do the people suppose that they will be let off because they say, ‘We have faith,’ and they will not be tested? Certainly, We tested those who were before them. So, Allah shall surely ascertain those who are truthful, and He shall surely ascertain the liars.” (29:2)

Prophet Abraham (PBUH) was a great Prophet of Allah (SWT) who is greatly celebrated in the Holy Quran. He is one of the Ulu –l-Azm (Arch) prophets. Allah (SWT) calls him his friend in the Holy Quran (4:125). The intended sacrifice of this holy prophet is greatly eulogized in the Holy Quran and the prophetic traditions. Abraham is especially famous for the numerous ways God tested him; especially the tough tests of leaving his wife Hagar and his firstborn son Ishmael (2:124) and bringing his son to a special place as an offering to God (37:100-113).

The story of Prophet Abraham sacrificing his son is mentioned in the Holy Quran as follows: 

“My Lord! Give me [an heir], one of the righteous.” So We gave him the good news of a forbearing son. When he was old enough to assist in his endeavor, he said, ‘My son! I see in dreams that I am sacrificing you. See what you think.’ He said, ‘Father! Do whatever you have been commanded. If Allah wishes, you will find me to be patient.’ So, when they had both surrendered to Allah’s will, and he had laid him down on his forehead, We called out to him, ‘O Abraham! You have indeed fulfilled your mission! Thus, indeed do We reward the virtuous! This was indeed a manifest test.’ Then We ransomed him with a great sacrifice, and left for him a good name in posterity: ‘Peace be to Abraham!’ Thus, do We reward the virtuous? He is indeed one of Our faithful servants.” (37:100-110)

The Holy Quran gives us a very clear picture of the incidents that unfolded with Prophet Abraham (PBUH). It tells us that Prophet Abraham (PBUH) prayed for a righteous heir and Allah (SWT) answered his prayers. But after a few years, Allah (SWT) orders his Prophet to sacrifice his only son and both Prophet Abraham (PBUH) and Prophet Ishmael (PBUH) agree to submit to the will of Allah (SWT). It was a test for Prophet Abraham (PBUH) as well as his son Prophet Ishmael (PBUH), who emerged victorious from this test. Thus, leaving a legacy of complete submission to the will of Allah (SWT), a submission that calls for sacrificing everything you have to gain the pleasure of your beloved creator. It is this humble and pure submission of Prophet Abraham (PBUH) that makes this great prophet a ‘good example’ for us. (60:4). 

The sacrifice of Prophet Abraham is not a fairy tale to be read to our children before going to sleep, it is a story that has the potential to make human beings build a lasting relationship with Allah (SWT). If we truly trust in Allah (SWT) and reach a position where we readily sacrifice everything in the way of Allah, we can proudly claim to be inheritors of the legacy of Abraham, a friend of Allah (SWT). But for us to reach that position we need to first ask ourselves: who is our Ishmael? Is it my child, my spouse, my property, wealth, job, position or my desires? 

References

1. Quran Translations, Ali Quli Qara’i
2. Recommended rituals and practices on the eve of Eid and the day of Eid: Dua for EID ul Adha Night and Day (duas.org) (these recommended acts can also be found in Mafatih Al-Jinan under the section of aamaal for Eid)
3. Exegesis of Surah Saaffat, Tafseer Nemuneh, Ayatollah Makarem Shirazi (Farsi)