Eid Al-Zahra ( 9th Rabi'ul Awwal) is one of the big days of happiness for Ahlul Bayt (AS). It is mentioned as the second Ghadeer. It is the day of the end of one of the main enemies of Lady Fatimah Al-Zahra (SA). In Quran we read that the believers celebrate and rejoice the end of the enemies of Allah (Sura 30, Verse 4).
Hadeeth of Ahmad Ibn Is-Haq al-Ash'ari from from Imam Hasan Al-Askari (AS) stated that the Prophet Muhammad (SAWA) has informed his grandsons Hasan and Husain to celebrate this day because it will be the end of their mother's enemy. ( Dala'il al-Imamah by ibn Jareer al-Tabari and Rawdhaat al-Jannaat by al-Khonsari, page 768.
Wassalam.