إِنَّ اللَّهَ عَزَّ وَ جَلَّ يَقُولُ وَ لِتُكْمِلُوا...
Allah -azwj the Mighty and Majestic is Saying [2:185] and (He Desires) that you should complete the number ; Shawwal is of twenty nine days and Zilqaada is of thirty days due to the Words of Allah -azwj the Mighty and Majestic. [7:142] And We Appointed with Musa a time of thirty nights and Completed them with ten (more), so the Appointed time of his Lord was complete forty nights ; Zilhijja is of twenty nine days and Al-Muharram is of thirty days.
Muhammad Bin Yahya, from Muhammad Bin Al Husayn, from Ibn Sinan, from Huzeyfa Bin Mansour, from Moaz Bin Kaseer, (It has been narrated) from Abu Abdullah -asws having said: ‘The Month of Ramazan is of thirty days, not being deficient, by Allah -azwj , ever!’ [12] Eid Salat is Void without the presence of the Just Imam -asws The majority of the 12 Imami Shias, get together on the Eid-day to offer Salat in Jamaat (congregational Prayer) behind a mullah, regardless of whether he is Fasting or not, depending upon the instructions he would receive from his Maraja.
So, he leads Eid prayers, usually on several days (in the West), sometimes even while Fasting!! However, the Eid Salat in Jamaat (congregational) is invalid without the presence of the Just Imam -asws (to have his -asws Government established over people), see for example one Hadith here, the additional Ahadith are given in Appendix II.
وروي زرارة بن أعين عن أبى جعفر عليه السلام قال: ” لا صلاة يوم الفطر والاضحى مع إمام عادل Zrara Bin Ain narrates from Imam Abu Jafar -asws that Imam -asws says:” There are neither Salat on the Day of Al-Fitar nor on Al-Azha except in the presence of a Just Imam -asws .