There has been multiple really great moments during the development. Its always cool to see some completely new features working on real hardware, like first time getting keyboard input, USB mouse input, running DOOM, reading from disks, networking, and getting a compiler working!

I think it became actually fun after I got all the basic requirements done and could start deviating from more or less standard OS code. When I could actually decide what I want to work on. Maybe after couple of months it was in that state. It was also fun at the beginning. it was just way harder as I didn't have any prior knowledge and I was trying to wrap my head around some basic concepts :D