Takes places in a theater troupe or on a film set
Play in question is Julius Caesar
Run/filming starts in March
There's a montage of scenes of actors and stage hands/set workers ribbing the actor who plays Caesar about the upcoming 15th. He laughs it off at first; then, as everyone goes on he scoffs about it, and after a while gets really angry.
On the 15th, and during That Scene, as the mob of senators rush Caesar, turns out several of the prop daggers aren't prop daggers
The actor who plays Caesar dies
Due to Really Good Stage Blood, none of the actors who play the senators know if they struck any actual blow, much less the fatal one.
Or at least they aren't saying if they felt their knife go in
Even if it's a film production, there aren't enough shots of the scene to tell, either
Practically everyone on set has a motive; writer's choice of what kind of asshole JC's actor was (you could take it in the direction of Murder on the Orient Express if you wanted, even!)
(side plot that due to the murder, interest in the play/movie has SKYROCKETED, causing at least one of the sleuths to shake their head in despair over the state of the world)
Bonus points if the denouement is delivered in Shakespeare meter (I don't know what it's called, sorry, like, iambic pentameter or whatever)
Title would obviously be Beware The Ides Of March.