To answer your burning question directly: Susannah dies between Season 1 and Season 2. The show reveals her death in the very first episode of Season 2. You’ll get a kick in the feels before the opening credits even finish.
But here’s the thing. Susannah’s death isn’t the point. The point is the love she leaves behind. It’s messy, it’s sad, and it’s a little bit funny because the Fisher boys really, really struggle to fold laundry without her. So grab your tissues, call your mom (or your friend who acts like a mom), and hit play. You’ve got this. And hey—if Belly can survive it, so can you.