20th Safar is the day of Arba'een, the 40th day after the martyrdom of Imam Hussain (a.s.), his friends and relatives in Karbala on 10th Muharram.
According to Imam Hassan al-Askari (A) today it is highly desirable to:
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Pray 51 Rakat namaz during the 24 hours of the day of Arba'een, (17 rakats in Wajib (obligatory) and 34 rakats in Naafilah Salaats.)
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Be in Karbala for Ziyarat of Imam Husain (A) and other martyrs, and if it is not possible, to recite Ziyarat of Imam Husain (A) and the other martyrs of Karbala from wherever you are.
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To put the forehead on the earth (preferably earth of Karbala) in prostration.
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To pronounce "Bismillah Hir Rahman Nir Raheem" in clear and loud voice while praying the Salaat.
Imam Jafar al-Sadiq (A) has advised the faithful to recite the Ziyarat of the day of Arbaeen:
After the recitation of Ziyarat, pray two rakat Salaat.
[Please refer to Mafatihul Jinan, Supplications and other Ziyarat books for Ziyarat of Arba'een.]
source : http://www.ashura.com