Significance of Arba'een and the Performance of the Ziyarat of Arba'een
Source: http://www.sibtayn.com/
Imam Hasan Al-Askari (AS) 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 (AS) 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 (AS) states that the heavens wept over Imam Hussain (AS) 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 (AS).

But why is it that we mourn and remember Imam Hussain (AS) 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 (AS) 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 (AS) 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 (AS), we hope and pray that this forty days of remembrance of Imam Hussain (AS), brings about a transformation in us by which we can continue to follow the path of Imam Hussain (AS) 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