Eid-e-Zahra (sa)

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

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]

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."

http://www.marefateahlebait.com/miscellaneous/9-rabi-al-awwal