Ides of March
The Ides of March is a day on the Roman calendar that corresponds to March 15. It was marked by several religious observances and became notorious as the date of the assassination of Julius Caesar in 44 BC. Ref. Wikipedia.
I know that the 15th of March is famous for this one event. To see a great leader be brought down in this manor was the beginning to Romans end. After the Senate had Caesar assassinated it made everyone understand that power can corrupt anyone and thus no one was safe.
Rome lasted a long time after this but it was never the same. Up until this point the Senate held the power and although there were times when one person amassed great power the Senate always managed to get the power back. After this and the victory of Caesar's nephew over Mark Antony and anointing himself Caesar (Where the terms Kaiser and Tsar come from) the Senate never held the power again, and Rome was no longer a Republic… it was an empire led by an emperor. Each Caesar in turn amassed more power and many increased her size.