Number forty is mystic in Theosophy. According to the Islamic culture if someone practices a good deed constantly during forty days, it would be his inseparable attribute and lead to descending of God's blessing. In some religions forty days and forty nights praying has been special position.

When Prophet Moses prayed forty nights; found the ability to hear the words of God, "And when We appointed a time of forty nights with Musa, then you took the calf (for a god) after him and you were unjust." [Holy Quran 2:5
Holy prophet Muhammad (SAW) said, "Whoever dedicates himself to God for forty days, will find springs of wisdom sprout out of his heart and flow on his tongue."

Commemorating of the fortieth day of deeds by their family and giving alms is one of the common customs among some of the Muslims. The fortieth day of Imam Hussain 's martyrdom is called Arbaeen. In this day Shiites commemorate Imam Hussain's revolution. These forty days are a suitable opportunity for people to develop the love of Imam Hussain and hate his murderers, in their hearts. Forty consecutive days, from Ashura up to Arbaeen, also is a national ceremony of aversion announcement from oppressors of the history.

In the first Arbaeen, Jabir Ibn Abdullah was the person who pilgrimage the holy tombs of Imam Hussain . According to some narrations, captive family of Imam Hussain , in the way of returning to Medina from Sham, visited Jabir and pilgrimage tombs of Ashura's martyrs.

O Hussain, commemoration of your revolution from Ashura up to Arbaeen makes the world pay attention to the message of the resurrection. Ashura is martyrdom day of the history makers and Arbaeen is pilgrimage day of Ashura makers.

Arbaeen philosophy shows itself, and we can teach martyrdom lesson from this class and prepare ourselves in order to help our master Hazrat-e-Mahdi (May God hasten his reappearance).
Arbaeen Aamal and Ziyarat:
20th Safar is the day of Arbaeen, the 40th day after the martyrdom of Imam Hussain , his friends and relatives in Karbala on 10th Muharram.

According to Imam Hassan al-Askari today it is highly desirable to:

1. Pray 51 Rakat Namaz (Salaat) during the 24 hours of the day of Arbaeen, [17 Rakats in Wajib (obligatory) and 34 Rakats in Naafilah].
2. Be in Karbala for Ziyarat of Imam Hussain and other martyrs, and if it is not possible, then recite Ziyarat of Imam Hussain and the other martyrs of Karbala from wherever you are.
3. To put the forehead on the earth (preferably earth of Karbala) in prostration.
4. To pronounce "Bismillah Hir Rahman Nir Raheem" in clear and loud voice while praying the Salaat.

Imam Jafar al-Sadiq has advised the faithful to recite the Ziyarat of the day of Arbaeen. [Please refer to Mafatihul Jinan, Supplications and other Ziyarat books for Ziyarat of Arbaeen.

After the recitation of Ziyarat, pray two Rakat Salaat.

IslamicOccasions.com-Arbayeen

Duas.org-Arbyaeen

Why Arbayeen?-Duas.org

http://hussainiat.com/