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)

Lady Zainab (SA) is an icon of purity, bravery, and steadfastness.Though she lived 14 centuries ago, her personality and teachings continue to guide men and women till now/to this day. She is the fruit of the tree of Prophethood nourished by Imamat. Lady Zainab (SA), the daughter of Imam Ali (As), has several attributes and titles associated with her, such as Al-Aqilah the noblewoman, Al-Aalimah or the knowledgeable woman and Aminatu-al-Allah, the faithful of Allah (SWT).
She belonged to a holy lineage; her mother was Lady Fatimah al-Zahra (SA), - the daughter of Prophet Muhammad (PBUH & HP) - and was actually a reflection of her father in the worship of Allah, piety, self-discipline, virtues, clemency, veneration, and the other features of perfection thus her father named her the chief of the women of the worlds and such a great mother nurtured Lady Zainab. 

Lady Zainab’s life

Lady Zainab’s father, Imam Ali (AS) honoured as the representative, successor and heir to Prophet Muhammad (PBUH&HP) was given remarkable titles like ‘the chief of the prophets’ successors’, ‘leader of the pious ones’ and ‘the head of the worshippers’ by Prophet Muhammad (PBUH&HP) himself.  With innumerable merits and unrivalled sacrifices for Islam he was recognised and reputed as the most knowledgeable, the most magnanimous, the most generous, the most abstinent, the most trustworthy, the best worshipper, the most faithful, and above all, the most pious and the bravest of all Muslims. In other words, he was the best Muslim after the Holy Prophet Muhammad. (PBUH&HP).

Lady Zainab  (SA), hence, was  born into the most pious family and raised  under the care of the best human beings therefore she learnt many things that composed her perfect personality from the etiquettes of her parents and her brothers.  Since early life, she was marvellous in her intellect and wisdom and had learnt the Holy Quran by heart as well as the Prophet’s sayings regarding Islamic laws, rules of education and principles of ethics.

Throughout her life she faced one tribulation after another, but emerged strong and victorious in all her trials. She then witnessed the martyrdom of her mother, father and brothers - Imam Hasan and Imam Hussain (AS) - and in Karbala where all male members of her family were brutally martyred including her own sons Awn and Muhammad, and all the women folk were taken into captivity. But despite all these vicissitudes she always remained a source of strength and a caretaker for the survivors of the tragedy of Karbala and the flagbearer of the message of this great event. She is the epitome of selflessness and perseverance, the embodiment of patience and courage in the face of oppressors and the personification of knowledge and purity. Despite all the hardships she endured in Karbala she decided to preserve and propagate the true message of Karbala While her bravery, eloquence, courage, asceticism, chastity and caring nature towards the survivors of the tragedy of Karbala has been and still is unparalleled in history. 

The Holy Prophet Muhammad (PBUH&HP) says, “Whoever tries to fulfil the needs of a sick person, whether they fulfil it or not, they will be cleansed of their sins like the day they were born.”[1] In another tradition,   he has said, “Whoever looks after the sick for an entire day, Allah (SWT) will raise them up with Prophet Abraham.”[2] If we would take only these two traditions into consideration, we can understand the greatness of Lady Zainab (SA) who cared for Imam Sajjad (AS) and all the children and women who had got sick or were injured after what happened in Karbala. Therefore, taking inspiration from her, the 5th of Jamadi-Al-Awwal which is her birthday is celebrated as the Nurses’ Day in the Islamic calendar. 

References:

1.  Kitab Man La Yahzarul Faqih, V.4, Pg.16 
2.   Sawab ul Aamal, Pg. 341
3.    https://www.al-islam.org/lady-zaynab-badr-shahin/life-lady-zaynab 
4.  https://www.al-islam.org/victory-truth-life-zaynab-bint-ali-muna-haeri-bilgrami