Probably the explanation for this matter may be the flood in Poznan that took place (according to the „Chronicle of Poznań city writers”) in Lent of 1612. Lent in that year began with Ash Wednesday on 7th march!
This chronicle says that the water was so high that submerged all the city bridges.
Maybe the chronicle itself would give more information, I’ve read only this article.
An explanation, that the documents were created later and a particular day (the beginning of Lent) was chosen as a death day, sounds reasonable.
Frankfurt and Poznań are some 800 km apart, were situated in different kingdoms, and it seems that the Fettmilch Rebelion was a local event.
The omniscient wikipedia says that the fires in Poznań started usually in the Jewish quarter or in suburbs and were spreading further to the city. The houses in that quarter were wooden in contrary to the brick houses inside the city walls.
We can believe that the fires started because of antisemitism, but remember that in realities of that time such antisemitism would end on the gallows as with the case of Fettmilch. It is far less risky to set a fire these days.