Is it true that Imam Hussein’s assassin was killed on the ninth of Rabi’ul Awwal and his head was sent to Imam Sajjad (a.s.)?

What is the historical importance or significance of 9th of Rabi\'ul Awwal? It is said that this was the day when killers of Imam Hussein (a.s.) were killed by Ameer Mukhtar and a letter was sent to Imam Zainul Abideen causing him to smile and so on?
Concise answer

The 9th of Rabi’ul Awwal can be studied from several angles:
First: There is a narration which describes this day as a day of great Eid or festivity for Shiites’ to the extent that the angels have been commanded to cease recording sins of Shiites on that day. As said in index “abolition of obligation on ninth Rabi’ul Awwal”, question 20229, such a narration or understanding is incorrect.
Second:  Another subject that has been associated with this day[1] is the death of Umar bin Saad, the commander of Yazid’s army against Imam Hussein, peace be upon him. It has been said that he was killed by Mukhtar on this day and that Mukhtar sent Umar bin Saad’s head along with a letter to Imam Ali (a.s.)’s son, i.e. Muhammad bin Hanafiyah. In this regard, we have to clarify a few points:
1. Mukhtar launched his uprising in the month of Rabi’ul Awwal[2] or Rabi’ al-Thani 66 A.H. The people of Kufah vowed allegiance to him and thereafter he started to punish the murderers of the martyrs of Karbala some of whom he did kill.[3]
Among those people who were brought to justice and killed in the year 66 A.H. were Shimr Zil Jawshan[4], Khuli bin Yazid[5], Ubaidullah bin Ziad[6] and Umar bin Saad[7].
2. Based on reports in history books, Mukhtar killed Umar bin Saad and sent his head along with some money to Muhammad bin Hanafiyah. Seeing Umar bin Saad’s head, Muhammad bin Hanafiyah prayed for Mukhtar as such: “O Allah, grant Mukhtar the best of rewards on behalf of Muhammad and his Ahlul-Bayt.”[8]
Indeed, according to another tradition which has been related by Kashi in his book, it says: “When Ubaidullah bin Ziad and Umar bin Saad’s heads were brought to Imam Zainul Abedeen (a.s.), the Imam prostrated praising and thanking Allah and praying for Mukhtar and wishing him well.”[9]  However, based on a another report by Ya’qubi,[10] Ubaidullah bin Ziad’s head was sent to the fourth Imam, Imam Sajjad, peace be upon him, and Umar bin Saad’s head was sent to Muhammad bin Hanafiyah.
3. Considering the historical reports in the sources, ultimately only two heads were sent to Imam Sajjad (a.s.) one of which was Ubaidullah bin Ziad’s head and the other was that of Umar bin Saad.
The conclusion is that, considering the reports and what was said above, Umar bin Saad’s head who was killed in the year 66 was sent to Muhammad bin Hanafiyah, because Umar Saad was killed in the same year.
Keeping in view the fact that Mukhtar launched his uprising in the month of Rabi’ul Awwal or Rabi al-Thani of the year 66, obviously Umar bin Saad could not have been killed on the nineth of Rabi’ul Awwal of the same year. Considering the distance between Iraq and Hijaz, it is difficult to accept that his head was sent to Imam Sajjad (a.s.).  Yes, it can be said and conceded that Umar bin Saad’s head was sent to the Imam but it is not true to say that he was killed on 9th of Rabi’ of the year 66 A.H.
As for Ubaidullah bin Ziad’s head which, according to some reports, was sent to Imam Sajjad, the same objection could be raised not to mention the fact that in some history and textual resources such a claim in respect of the ninth of Rabi’ul Awwal is not proven.
 
[1] It has been stated on some websites as such.
[2] Miskawayh Razi, Abu Ali, Tajarub al-Umam, researched: Imami, Abul Qasim, vol.2, p. 146, Soroush, Tehran, 1379 (2000).
[3] Shaykh Tusi, Muhammad bin hasan, Amali, p. 240, Dar al-Thaqafah Publications, qom, 1414 A.H.
[4] Ibn Kathir, Ismail bin Umar, Al-Bedayah wa al-Nihayah, vol.8, Dar al-Fikr, Beirut, 1407 A.H.
[5] Ibid, p. 272.
[6] Mas’udi, Abul Hasan Ali bin al-Hussein, Murawwij al-Zahab wa Ma’aden al-Jawhar, researched by Daghar, As’ad, vol.3, p. 97, Dar al-Hijrah, Qom, second edition, 1409 A.H.  Of course, after quoting the said saying about the death of Ibn Ziad in the year 66 A.H., he says: What is widely known is that he was killed in the year 67 A.H. Vide: Al-Bedayah wa al-Nehayah, vol.8, p. 286.
[7] Al-Beyah wa al-Nehayah, vol.8, p. 273.
[8] Ibn A’tham Kufi, Ahmad bin A’tham, al-Fotuh, researched by, Shiri, Ali, vol.6, p. 247, Dar al-Azwaa, Beirut, 1411 A.H; al-Bedayah wa al-Nehayah, vol.8, p. 274; Balazari, Ahmad bin Yahya, Ansaab al-Ashraf, researched by Zakaar, Suhayl, Zarkali, Reyadh, vol.6, p. 406, Dar al-Fikr, Beirut, 1417 A.H.
[9]  The Arabic version of the report is as under:
[9]. «أَنَّ عَلِیَّ بْنَ الْحُسَیْنِ(ع) لَمَّا أُتِیَ بِرَأْسِ عُبَیْدِ اللَّهِ بْنِ زِیَادٍ وَ رَأْسِ عُمَرَ بْنِ سَعْدٍ خَرَّ سَاجِداً وَ قَالَ الْحَمْدُ لِلَّهِ الَّذِی أَدْرَکَ لِی ثَأْرِی مِنْ أَعْدَائِی وَ جَزَى الْمُخْتَارَ خَیْراً»
Kashi, Muhammad bin Umar, Ikhtiyar Ma’refat al-Rejal, researched and edited: Shaykh Tusi, Muhammad bin Hasan, Mustafawi, Hasa, p. 127, Mashad University Press, first edition, 1409 A.H.
[10] Yaqubi, Ahmad bin Abi Ya’qub, Tarikh Yaqubi, vol.2, p. 259, Dar