Why is the martyrdom of the daughter of the Prophet, Fatima al-Zahra, commemorated at three different times?

In the Shia school of thought, the martyrdom of the daughter of the Prophet is commemorated at three different times; the last one today, on 3rd of Jumada al-Thani. Why is it that "there are three different times for the Wafat of Fatima al-Zahra"?