Arbaeen is one of the most important event in the Islamic history; therefor, this article is going to briefly deal with the importance if this day in the Islamic world.
On the occasion of Arbaeen (20th Safar) many traditions have been narrated from the infallible imams (A), according to which Muslims must observe some certain rules and rituals in this holy day. According to Imam Hassan bin Ali Al Askaree (as) on the day of Arbaeen it is highly recommended to: 1) Pray 51 Rak—a’t Namaaz during the 24 hours of the day of Arbaeen. 2) Be in Karbala for Ziyaarat of Imam Husain and other martyrs, if not possible, recite Ziyaarat of the Holy Imam and other martyrs of Karbala. Imam Jaa’—far bin Muhammad Al Sadiq (as) had advised the faithful to recite the “Ziyaarat Arbaeen” on the day of Arbaeen. 
Ziyaarat, which means visitation, is not necessary to be performed in Karbala, rather Ziyarat of Imam Husain (A), far from Karbala, would hold the same significance as being in the land of Karbala. But the important point is that the performer of the Ziyarat must truly understand the status of the Imam and seek to emulate him.
From the traditions of the Holy Prophet (P) and other infallibles (as) it could be deduced that the number “forty” itself holds some spiritual significances. For instance, the holy prophet of Islam says: “The earth mourns the death of a believer for forty mornings.”  Imam Muhammad al-Baqir (as) states that the heavens wept over Imam Husain (as) for forty mornings, rise red and set red.  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 (as).  Visitation of the shrine of Imam Husain (as) as well as Masjid-e-Sahla for 40 consecutive Thursdays is also very highly recommended.
Following an infallible imam for forty days helps you to mold your lives according to the teachings of the imams, and you pledge that you will continue follow his right path. In addition, Praying for a deceased Muslim, for forty days, will cause the forgiveness of Allah the exalted upon him. Thus, as we perform the Ziyarat of Arbaeen and commemorate the Arbaeen, 40 days of mourning of Imam Hussein (as).
Finally it must be mentioned that we hope and pray that this forty days of remembrance of Allah , his prophets, and Imam Hussein (as), brings about a transformation in us by which we can continue to follow the path of Islam and the prophet Mohammad (P). in fact this Pilgrimage is the best chance to remember Allah and perform our Islamic duties in such a divine and sacred place.
 Said Ibn Tawoos, Eqbal-al-Aamal, v3, p. 100.
 «مصباح المتهجّد» جزء ثالث، ص 730 (في فضيلة زيارة الأربعين)
 طبرسی، مکارم الاخلاق، ص 465
 كامل الزیارات، ترجمه ذهنى تهرانى، حدیث 6، ص 258.
 مجلسی، بحارالانوار، ج۸۳، ص۲۸۴.