I think it all depends. If it was everyday your body would likely go into starvation mode and it could potentially damage your BMR and cause unwanted metabolic adaptions as well as malnutrition. If it was every once and a while I don’t think it’s horrible but it might lead to over eating on later days. If you have nutrition questions you should talk to a Dietitian ( if the the US). They can help you determine what would be best for your health!
Once a week? Assuming you’re use to it. It’s fine, would be less effective and not as healthy comapared to fasting (because your body stays in the swing of things). If you just have 500 calories you risk spiking and crashing energy.
Benefits would include gut health (less to process, it takes a break from the constant digestion). Calorie deficit if you’re needing that. Autophagy (destruction of old cells, burned up for energy, associated with longevity) increased.
I would highly recommend you look to YouTube, specifically SIIM who is a big brain in this type of stuff.