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?
There can be diverse reasons for that, such as:
And so on.
It's because of the ambiguity. We don't know exact time of martyrdom. Among of several guessed time from history-writers these three times have more logical proofs.
Tip: search for the way hadzrat Fatima has martyrdomed, and you find out.