Today is the day for the big reveal, the big battle, the solving of the mystery, the moment that your MC and the readers have been waiting for. It’s time for the protagonist and antagonist to meet one on one, for the villain and hero to face each other, and for everything your story has been working towards to culminate. Today, write the climax of your story.
Blake Snyder refers to this moment as “the Finale.” It’s the moment where “the main character incorporates the Theme – the nugget of truth that now makes sense to them – into their fight for the goal because they have experience from [Act 1] and context from [Act 2]. Act Three is about Synthesis!” (Tim Stout, “Blake Snyder’s Beat Sheet”)
Remember to incorporate the theme of your story into this moment, and to reveal (or have your MC discover or utilize) the missing element he/she has been searching for throughout the entirety of the journey.