If there is no evidence that our beloved Prophet (PBUH) celebrated the birthday of his grandsons, does it imply that we should also avoid celebrating birthdays of our twelve Imams?
It is authentic in both Sunni and Shia books that the Prophet Muhammad (SAWA) celebrated the birth of his grand children, Hasan, Husain and Zainab.
Celebrating the birth anniversary of the twelve Imams is highly recommended as it reminds us with the great teachings of Islam for which the whole life of the twelve Imams was dedicated and sacrificed.
Wassalam.