Eastern Orthodox Easter is May 4th, 2024.
I had to search on what battle it was - the Battle of Ulm. However, several sites indicate that is just a myth. Below is a good refutation of it being a simple mistake of the date.
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http://dcjack.org/kagan%20on%20ulm.html
Excerpt:
Dear Mr. McKay,
I'm happy to have an opportunity to think about 1805, thank you! I thought I had addressed that issue in a footnote, but perhaps not.
It is a myth. I have seen with my own eyes the march-plans the Austrian general staff developed showing where the Russian forces would be on each day—and using the right calendar. And I have seen no evidence whatever in the voluminous correspondence between the Russians and the Austrians and within the Austrian army and court that anyone was confused about this. It is a bizarre myth, particularly considering that Russian and Austrian armies had been fighting in close proximity for many years both against France and against Turkey, and all Russian correspondence directed to non-Russian recipients carried both dates as a matter of course.
I think that there is a contemporaneous French source that mentions this, and, of course, David Chandler picked it up in his Campaigns of Napoleon. But it is entirely without foundation.
Regards,
Fred Kagan
Why is Easter on a Saturday?
Cool.