I’m going to presume you already know what Day One is. If not I’m not going to go beyond saying it’s a journaling app, so go check it out and come back. Then I can get to the good stuff!
I’ve used Day One on and off since it was originally released. I’ve always loved using it, but haven’t always been able to find the perfectly-fit itch that I needed to scratch with it until the last year or so. Generally speaking I don’t do open journaling very well so a more structured approach has worked well for me. It’s also gamified portions of it which adds its own sort of fun!