Bismihi ta'ala
There are views that Imam Musa al-Kadhim (a.s.) was born on the 7th of Safar, and that this day was also mentioned as the date of the martyrdom of Imam Hasan al-Mujtaba (a.s.). The different views on the martyrdom date of Imam al-Mujtaba (a.s.) can be discussed another time, so we will focus on the birthday of Imam al-Kadhim (a.s.) and the different views.
Firstly, early scholars like Sheikh al-Kulayni, Sheikh al-Mufid and Sheikh al-Tusi did not really clarify this issue. They did mention the date of Dhil Hijjah.
It is said Imam al-Kadhim (a.s.) was born in the year 127 AH (or 128, or 129 AH), in an area by the name al-Abwa`, which was between Meccah and Medinah, in Dhil Hijjah, where Imam al-Sadiq (a.s.) was on his way back to Medinah, after Hajj pilgrimage.
The date given for this is 20th of Dhil Hijjah.
There are narrations for this, and Ibn Jarir al-Tabari mentions this in his history book as well.
One argument against 7th (or even 9th) of Safar date is what was Imam al-Sadiq (a.s.) doing in that area of where Imam al-Kadhim (a.s.) was born? It was not a custom for them to stay in Meccah from Hajj for that long or to travel during that time.
Another very important point is the conduct of 'Ulama and their statements. It is usually customary in the Hawzah that 7th of Safar is a mourning day, and Hawzah classes are cancelled for 'azaa for Imam al-Mujtaba (a.s.).
Contemporary Maraji' and 'Ulama, like Seyyid al-Sistani, Sheikh Wahid Khurasani, Sheikh Safi Gulpaygani, Seyid Shubayri Zanjani, Sheikh Bahjat, and many more, say 7th of Safar is martyrdom anniversary of Imam al-Mujtaba (a.s.), and it is a mourning day.
As for the view that Imam al-Kadhim (a.s.) was born in Safar, for those historians and scholars who take this view, it is absolutely certain that we put mourning over celebration, and so the merits of Imam al-Kadhim (a.s.) can be mentioned, with the tragedy of the martyrdom of Imam Hasan al-Mujtaba (a.s.).
And Allah knows best.
