It’s hard to say without knowing your age, height, etc. 900 calories in a day are not sustainable for almost anyone, though. Make sure you find your BMR (basal metabolic rate), consider your activity level (exercise, etc.) and try to get the calories and nutrients your body needs.
Thats super low, as long as its not an every day thing though, you’re fine. I find if I have a lot Fri/Sat (I don’t track those days, but loosely keep to an IF schedule), by the time I hit 18hrs Sunday and have my normal lunch, I don’t wind up having dinner cause its so late in the day