Julius Caesar was famously assassinated on the Ides of March (March 15th) in 44 BC. While it is impossible to know exactly what he should have said, an ideal statement might have been something that could have alerted him to the conspiracy or prevented his assassination.
Here's a fictional quote that could fit the situation:
"Friends, Romans, and Senators, let us speak openly and honestly about our differences today.
I invite you to share your concerns, so that we may build a stronger and more united Rome.
On this Ides of March, let us choose unity over division, for only together can we truly prosper."
--Julius Caesar March 15th, 44BCE K9
This quote encourages dialogue, unity, and understanding, which might have potentially averted the tragic event.
However, it is important to note that this is a hypothetical statement, and the actual historical events cannot be changed.