9th of Rabi Al-Awwal the day of Eid:

For the Complete narration of Eid-e-Zahra (sa) ... read here

Eid-e-Zahra (sa) is also called as Eid al-Baqr .. But Why ???

The Sunni scholar Ibn Abi al-Hadid in his book Sharh Nahj al-Balaghah says:

إن أبا بكر رق لها حيث لم يكن عمر حاضرا فكتب لها بفدك كتابا فلما خرجت به وجدها عمر فمد يده إليه ليأخذه مغالبة فمنعته فدفع بيده في صدرها و أخذ الصحيفة فخرقها بعد أن تفل فيها فمحاها و إنها دعت عليه فقالت‏

After Abu Bakr and Umar refused to give Fadak to Fatema Zahra (sa) and then Abu Bakr while Umar was not present, gave Fadak to Her by writing on a piece of paper. Then Umar met Fatema Zahra (sa) and snatched the paper from her and tore it. At that moment Fatema Zahra (sa) said to Umar;

بقر الله بطنك كما بقرت صحيفتي

"May Allah (swt) tear open your stomach like how you tore my paper"

[Source: Sharh Nahjul Balagha - Ibn Abil Hadeed, Vol. 16, Pg. 235]

Sheikh Abbas Qummi in Mufati al-Jinaan regarding 9th of Rabi Al-Awwal:

اليوم التّاسع : عيد عظيم وهو عيد البقر وشرحه طويل مذكور في محلّه وروي انّ من أنفق شيئاً في هذا اليوم غفرت ذُنوبه وقيل يستحبّ في هذا اليوم اطعام الاخوان المؤمنين وافراحهم والتّوسّع في نفقة العيال ولبس الثّياب الطّيّبة وشكر الله تعالى وعبادته وهو يوم زوال الغُموم والاحزان وهو يوم شريف جدّاً

"The 9th of Rabi Al-Awwal: It is a great Eid and it is the Eid of Al-Baqr (referring to the splitting open of stomach of Umar?) and its explanation is long and is stated in its required place and its narrated that verily one who spends (nifaaq) any thing on this day will have his sins forgiven and its is said that its Mustahab in this day to give food to the believing brothers and to make them happy and to increase the spending on ones household and wear clean clothes and thank Allah (swt) and worship Him and this day is the day of passing of distress and grief and it is very noble day."

[Mufati Al-Jinaan - Shikh Abbas Qummi]

Explaining the meaning of Eid al-Baqr, Sayed Sadiq Rouhani says:

(البَقْر) مصدر بقر يبقر بقراً، والمراد منه يوم شقّ بطن أحد أعداء الزهراء عليها السلام، وهو الذي ظلمها وهجم عليها وعصرها وأسقط جنينها، ممّا أدّى إلى شهادتها كما وردَ ذلك مستفيضاً في كتب الفريقين وقد بُقِرَ بطنه في اليوم التاسع من شهر ربيع الأوّل على يد التابعي الجليل أبي لؤلؤة النهاوندي المدني، فيحتفي الشيعة فرحاً بهذا اليوم‏ويعبّرون عنه بعيد البقر ؛ لأنّهم يعتقدون أنّ اللَّه تعالى قد انتقم فيه للصدّيقة الزهراء عليها السلام ممّن ظلمها وهتك حرمتها، وذلك ببقر بطنه وتمزيقه

"The day of Al-Baqr refers to the day of cutting of the stomach of an enemy of Fatema Zahra (sa). And that enemy is the one who oppressed Her and attached Her and squeezed her and caused the falling of her Baby, resulting in her martyrdom, like has been reported extensively in books of both the groups (shia and sunni). And surely the stomach of that enemy was cut open on the 9th of Rabi al-Awwal at the hands of the great Taaba'ii, Abi Luluh al-Nahawandi al-Madani. So the Shias celebrate this day as the day of a Eid al-Baqr. Because they believe that Allah (swt) on this day took revenge for Fatema Zahra (sa) from those who oppressed her and offended her sanctity by cutting His stomach ripping it apart."

Source: www.rohani.ir/ar/idetail/7679/ما-معن---عيد-البقر--

Sayed Ibn Tawoos says regarding 9th of Rabi Al-Awwal:

اعلم أن هذا اليوم وجدنا فيه رواية عظيمة الشأن، ووجدنا جماعة من العجم والإخوان يعظّمون السرور فيه، ويذكرون أنه يوم هلاك بعض من كان يهون بالله جل جلاله ورسوله صلوات الله عليه ويعاديه

"Know that for this day we have found great narration. And we have found a group from the Ajam(Persians?) and our brothers (in faith) who showed happiness in this day and they recalled that verily this is the day of destruction of few of those who humilated Allah (swt) and his Prophet (saww) and were hostile to Him."

[Iqbal Al-Amal, Pg. 597]

Allama Majlisi quotes from Zwaid al-Fawaid by Sayed Ibn Tawoos after quoting the long narration on Eid on the day of 9th of Rabi Al-Awwal:

نقلته من خط محمد بن علي بن محمد بن طي رحمه الله، ووجدنا في ما تصفّحنا من الكتب عدة روايات موافقة لها فاعتمدنا عليها، فينبغي تعظيم هذا اليوم المشار إليه وإظهار السرور فيه

"And we have found in what we read from the books several narrations in consent with this narration (about Eid on the day of 9th Rabi Al Awwal). We rely upon it. We should respect this day being referred to (9th Rabi Al-Awwal) and show/display happiness in it."

[Source: Bihar Al-Anwar, Vol. 95, Pg. 351]

Sheikh Muhammad Husain Najafi in his book "Jawahir Al-Kalaam" says:

وأما الغسل للتاسع من ربيع الاول فقد حكي أنه من فعل أحمد بن إسحاق القمي معللاله بأنه يوم عيد

لكن المعروف الآن بين الشيعة انما هو يوم تاسع ربيع ، وقد عثرت على خبر مسندا إلىالنبي ( صلى الله عليه وآله ) في فضل هذا اليوم وشرفه وبركته وأنه يوم سرور لهم (عليهم السلام)

(فلعلنا نقول استحباب الغسل فيه بناء على استحبابه لمثل هذه الازمنة وسيما مع كونه عيدا لنا وأئمتنا (عليهم السلام

"And as for the Gusl for the 9th of Rabi-Al-Awwal, it has been narrated that it was the act of Ahmad bin Ishaq Al-Qummi , with the reasoning as it is the day of Eid."

"But it is famous till today among the Shias, that that is the day of 9th of Rabi-Al-Awwal. And it has been found in a narration reaching up to Prophet Muhammad (saww) about the virtue of this Day and its honor and its blessings verily it’s a day of happiness for Them (as)."

"And we say that the recommendation of Gusl on this day is based on the recommendation of it for such times (days) and especially with it being Eid for us and our Imam’s (as)."

[Source: Jawahir Al-Kalam, Vol. 5, Pg. 43-44]