@paninid I hate to "well, actually…" this because it's a funny joke, but well, actually…
Julius Caesar (the one who was stabbed) died 44 BCE. The change that messed up this happened about 100 years prior, when the Senate (Rome was a republic back then, there was no emperor) moved the start of the consular year (the start of the consuls' term) from the Ides of March to January. It was moved to January 1st so that Quintus Fulvius Nobilior could begin his term early.
https://en.wikipedia.org/wiki/Quintus_Fulvius_Nobilior
It's a common myth that Julius Caesar is somehow responsible for this because he was the one that introduced the julian calendar (the predecessor to the gregorian one we use today) and this officially made January the first month of the year, even if this had been the practice for about 100 years already. At this time the 5th and 6th months were called Quintilis and Sextilis. Quintilis was renamed to honor Julius Caesar after his death, and later Sextilis was renamed to honor Augustus Caesar.
I'm sure I'm making some mistake here, besides the obvious one of trying to correct somebody on the Internet 😉.