On which day do Mercutio and Tybalt die in the play "Romeo and Juliet?" I know how they die I just don't know what day... can anyone help?

We know that these deaths, which occur in Act III, Scene 1 of "Romeo and Juliet," occur on a Monday.

We know this because it occurs on the day after Romeo and Juliet have secretly been married.  This marriage actually helps to cause the deaths.  Romeo does not want to argue with Tybalt at first because he feels like they are now kinsmen.  But then Tybalt kills Mercutio while Romeo is trying to break up the fight.

So in this way, the day on which the killings happen is related to the plot.