From what I've read about starving yourself vs fasting and my own experience 800-calories sounds like HELL.

I can fast for 2 weeks no problem (although that's kinda long and I wouldn't do it if I didn't have enough body fat to sustain it. If you're skinny and don't have fat to spare better not stretch it unnecessarily).

I'm eating keto and I usually only eat once a day.

Fasting feels good. Starving yourself feel horrible.