Arbaeen rally is a example of unity and it is one of Islamic rituals
A’rbaeen is an Arabic word which means 40th day of Imam Husain’s martyrdom. Sheikh Tusi in his great book, Misbah al-Mojtahid in a chapter referring Safar month says it is when Jabir ibn Abdullah al-Ansari who is one of the Companions came from Medina to Karbala to visit the grave of Imam Husain a.s and he is the first one visiting Imam’s grave.
It is recommended to visit Imam’s tomb in Arbaeen, furthermore, he brings a narration from Imam Hassan Askari (a.s) which states the signs of the believer are five things:
1. Praying 51 raka’h every day (obligatory prayers and night prayers)
2. Visiting Imam Husain on the of Arbaeen (Ziaratul Arbaeen)
3. Wearing on a ring in the right hand
4. Ta’firul Jabeen (fogging forehead due to prostration)
5. Saying Bismillah-er-rahman-er-rahim loudly
These are the signs of whom they believed in Allah obeying His friends. In another verse Quran reads “Say, I do not ask you any reward for it except love of [my] relatives.” Imam Husain (a.s) is surely one of the best individuals and dear friends before Allah the Exalted. Therefore, one of the examples of Mavadat verse which is mentioned is the Arbaeen Event on which visitors from all around the world gather to gather to walk the rout of Najaf and Karbala in order to demonstrate that they are acting according the verse by showing firm friendship to Imam Husain (a.s).
There is another significant narration from Imam Reza which says “Allah may bless one who revives our work. The narrator asked: how does your work revive? Imam replied: by learning our teachings and teaching them to others. If people knew advantages of our teachings, surely, they would follow us.
Visiting Imam Husain (a.s) especially on Arbaeen is not only the symbol of reviving the Islamic teachings, but also, it is one of the sacraments of Allah which is mentioned in chapter Hajj: “That. And whoever venerates the sacraments of Allah indeed that arises from the God wariness of heart.”
This very point has mentioned in Tafsir Nemoune that every religious ceremony which reminds Allah and demonstrates the greatness of His rituals is the sacraments of Allah. It is absolutely clear that Arbaeen Event is one of the obvious and outstanding examples.
In the Arbaeen ceremony Shias are divided into only two groups, Zuvvar al-Husain, people who are to visit Imam Husain (a.s) and Khuddam al-Husain who are serving visitors of Imam Husain (a.s).
 Misbah al-Mojtahid. Shiekh Tusi. P 241
 42:23 قُلْ لا أَسْئَلُكُمْ عَلَيْهِ أَجْراً إِلاَّ الْمَوَدَّةَ فِي الْقُرْبی
 Mizan al-Hikma. V 8. Naration 13797عن الامامُ الرِّضا علیهالسلام: رَحِمَ اللّهُ عَبْدا اَحْیا أمْرَنا،»[قال الرّاوی:] فَقُلْتُ لَهُ: «فَکَیْفَ یُحْیِی أمْرَکُمْ؟ قالَ: یَتَعَلَّمُ عُلُومَنا و یُعَلِّمُها النّاسَ، فَإِنَّ النّاسَ لَوْ عَلِمُوا مَحاسِنَ کَلامِنا لاَتَّبَعُونا.
 22:32 ذَلِكَ وَ مَن يُعَظِّمْ شَعَائرِ اللَّهِ فَإِنَّهَا مِن تَقْوَى الْقُلُوب
 Tafisr al-Nemoune. V14. P 97