The Ides of March certainly sound great, but if we celebrated the anniversary of the famous assassination would it actually be March 15? The Julian calendar was only a few years old at the time, and it was centuries before leap days were fully accounted for. I've tried to find an answer, but haven't found any good resources on how the original Julian calendar worked, or how it compares with a modern calendar.
According to the best math I'm capable of with a calculator, and also an online converter between the Julian and Gregorian Calendars, Gaius Julius Caesar was assassinated on the 15th of March in our reckoning, too. The Julian calendar had not, it seems, slipped to the point it would when Pope Gregory reformed it.
This is the website I used to double-check: http://www.csgnetwork.com/juliangregcalconv.html
I think this is the crux of the question...
Does Jan 1 of the Julian calendar originally implemented coincide with the modern Jan 1 on the solar calendar... if it did, then Caesar was actually assassinated on March 15, because it didn’t have time to drift. However, I can't find anything supporting that synchronization existed in 45 BCE. Still digging through references though, so maybe somebody clever wrote a paper correlating astronomical research around the time with a modern calendar reckoning.