Imam Hasan Al-Askari (a.s.) includes the performance of Ziyarat-e-Arba’een as one of the five marks for a believer, in addition to reciting 51 raka of sala, audible recitation of Bismillah, wearing a ring on one’s right hand, and doing sajda on dust.  (This hadith has been narrated by Sheikh Tusi in the book Tahthib).
Why is so much importance given to the performance of the Ziyarat of Arba’een and the observance of Arba’een (40 days of mourning)?
Ziyarat, as we all know is a visitation, which in essence, is the act of speaking with and visiting our role models.  Of course the physical manifestation of the ziyarat is actually being present in Kerbala to perform the Ziyarat of Arba’een, but, in reality, that is not possible for every believer.  So does making ziyarat while not in Kerbala give one the same benefits?  Ahadith tell us that performing the Ziyarat of Imam Hussain (a.s.) far from Kerbala would hold the same significance as being in the land of Kerbala, as long as the person performing the Ziyarat has truly understood the status of the Imam and seeks to emulate him.
Imam Muhamed al-Baqir (a.s.) states that the heavens wept over Imam Hussain (a.s.) for forty mornings, rising red and setting red.  As we complete 40 days of remembrance of our Imam, we re-assert our pledge of obedience and loyalty to him,
“I bear witness that you are the Imam (who is) the upright, the pious, well-pleased (with Allah), the pure, the guide and the rightly-guided.
I bear witness that you fulfilled the pledge of Allah and you struggled in His way…I am a friend of whoever befriends him ...”
By performing the Ziyarat of Arba’een, we pledge that we will continue to follow the path of justice and righteousness and will reject injustice and speak up against the oppressors of the time,
I am an enemy of whoever is his enemy
In essence, we make a promise to continue to mold our lives according to the teaching of Imam Hussain (a.s.).
But why is it that we mourn and remember Imam Hussain (a.s.) for a period of forty days and then move on?
Our Prophet has said, “The earth mourns the death of a believer for forty mornings.”   Therefore it appears that the deceased should be remembered and mourned over for a period of forty days.  Performing an act for a continuous period of forty days is also known to help one to not just form a habit, but also to carry on the practice for the rest of his life.
If we look at studies dealing with bringing a lifestyle change, we notice the time period emphasized to bring about a change is usually 6 weeks which is about the same time period as 40 days (to be exact it is 42 days).
In the book 40 Days to Personal Revolution: A Breakthrough Program to Radically Change Your Body and Awaken the Sacred Within Your Soul, the author Baron Baptiste explains the significance of forty days:
Why forty days? Because the number 40 holds tremendous spiritual significance in the realm of transformation. Jesus wandered in the desert for forty days in order to experience purification and come to a greater understanding of himself and his mission. Moses and his people traveled through the desert for forty years before arriving at their home in the holy land. Noah preserved the sacredness of life by sailing his ark for forty days and forty nights. According to the Kabbalah, the ancient Jewish mystical text, it takes forty days to ingrain any new way of being into our system….
Reciting particular duas for a period of 40 days is highly recommended in our practices as well.
The unit 40 (forty) is said to be very effective. If a particular Duaa is recited 40 times, or 40 people gather to recite it, or it is recited for 40 days then its effectiveness is highly increased. (Duas.org)
It is reported from Imam Ja'fer Sadiq (a.s.) that whoever recites Dua-e-Ahad for forty days, after morning prayers will be amongst the helpers of the 12th Imam (a). Visitation of the shrine of Imam Hussain (a.s.) as well as Masjid-e-Sahla for 40 consecutive Thursdays is also very highly recommended and is one of the acts that promises a visit from the 12th Imam (ajtf).
Thus, as we perform the Ziyarat of Arba’een and commemorate the Arba’een, 40 days of mourning of Imam Hussain (a.s.), we hope and pray that this forty days of remembrance of Imam Hussain (a.s.), brings about a transformation in us by which we can continue to follow the path of Imam Hussain (a.s.) and carry on his message of upholding justice with true faith and a strong sense of sacrifice!
Refs:
A’maal of Muharram and Safar (Tayyiba Publishers)
Kerbala and Beyond by Yasin T. Al-Jabouri
The Miracle of Ziyarat-e-Ashoora
Commentary on Ziyarat Ashura, translated by Saleem Bhimji from the work of Ali Asghar Azizi Tehrani
Kitab al-Irshad, translated by IKA Howard, authored by Sheikh Al-Mufeed
Jabir's powerful words to 'Atiyyah on the Arba'in of Imam al-Husayn ('A)
Al-A’mash narrates from ‘Atiyyah al-‘Awfi, who said:
I left with Jabir bin ‘Abdillah al-Ansari (may Allah have mercy on him) to visit the grave of al-Husayn bin ‘Ali (‘a) bin Abi Talib (upon whom be peace); and when we entered Karbala, Jabir came near the bank of River Euphrates, performed the major ablution (ghusl) and wore his clothes; then he opened his purse which contained su’d [a plant with sweet odor]. He spread the perfume over his body; thereafter he did not take a step save in the remembrance of Allah, until he neared the grave [of al-Husayn ‘a].
[Then] he said to me:Make me touch the grave, and I made him do so. Thereupon he fell over the grave unconscious.
I sprinkled some water on him, and he gained consciousness.
Then he cried three times ‘Ya Husayn....’
Addressing al-Husayn (‘a) he said: 
Why is the beloved not responding to the call of the lover? And how can you respond, while your veins have been severed, and your body has been separated from your head? I bear witness that you are the offspring of Prophets, and the son of the master of believers, and the offspring of the companion of piety, and the offspring of guidance, and the fifth member of the people of the cloak, and the son of the master of chiefs, and the son of Fatimah, the mistress of women; how can you not be so, while the hand of the Leader of Apostles (s) fed you, and you were breast fed from the breast of belief, and trained in the bosoms of pious people, your weaning accompanied the state of utter submission; hence you enjoyed a pleasant state both during your life and death; however, the hearts of the believers are sorrowful due to your separation, and do not doubt about your good position. So upon you be Allah’s Peace and Approval. And I bear witness that you faced what your brother [Prophet] Yahya ('a) had faced.
Thereafter Jabir felt the presence around the grave and said:
Peace be unto you and the spirits that have descended in the neighborhood of al-Husayn (‘a)…I bear witness that you kept prayer steadfast and gave the zakat and you commanded the good and prohibited the forbidden and struggled against the deniers of religion and you worshipped Allah until conviction came to you.
I swear by the One Who sent Muhammad as a Prophet with truth; surely we participated with you in what you encountered.
 ‘Atiyyah (not understanding this powerful statement made by Jabir) says: I asked Jabir: How can that be possible, while we did not descend on any valley, nor did we climb any mountainous area, nor did we fight with the sword, whereas the heads of the group of al-Husayn (‘a) have been separated from their bodies, their children made orphans and their women made widows?
 Jabir said: “O ‘Atiyyah, I heard from my beloved, the Apostle of Allah (S), say:  ‘Whosoever loves a nation, would be resurrected with them, and whosoever loves the deed of a nation, would be considered one who has participated in the deed. I swear by the One Who Sent Muhammad as a Prophet in truth, surely my intention and the intention of my companions is according to the intention of al-Husayn and his companions.
 [Then Jabir says to ‘Atiyyah]: take me towards the houses of Kufa[n]. ‘Atiyyah says: when we reached a certain point on the way, Jabir said to me: ‘O ‘Atiyyah, may I advise you, for I do not think that I shall meet you after this short journey? Love the lover of Aali Muhammad as long as he/shes loves them, and have aversion for one who has aversion for Aali Muhammad as long as he/she has aversion for them, even if he/she were to fast and stand in prayer frequently; and befriend the lover of Aali Muhammad, for surely if one of their feet were to slip due to excess of sin, the other foot would be firm due to love for Aali Muhammad, for verily the lover of Aali Muhammad would return to Paradise and one who have aversion for them would return to Hell Fire.
 [Ref: 'Allamah Majlisi, Bihar al-Anwaar, v.65, p.131]