In Act III scene 1 of Romeo and Juliet, there are 3 possible climaxes as listed below. Which is the climax and why?
When Mercutio curses both houses, when Romeo says "this day's black fate," when Romeo says "I am fortune's fool!"
I think that it is important to look at the scene as a whole in order to identify the moment with the highest emotional tendency. Clearly, the three events you have quoted are related to each other. I would argue that Mercutio's dying speech and his curse on both houses and then Romeo's reference to the "black day" are just part of the rising action of this scene that finds its climax in Romeo's shouted lament of "I am fortune's fool!" Note what Romeo has just done before uttering this ejaculation. Mercutio having killed by Tybalt, Romeo was honour-bound to fight Tybalt in turn. Now that Romeo has killed Tybalt, which was something he did everything he could to avoid because of his marriage to Juliet and the fact that this makes them kinsmen, he recognises that he must run away from his wife. This explains why he feels like he is a plaything of fortune and that destiny seems to be acting against him.