Veteran here.

The first 3-3.5 days are, by far, the worst. The neuroscience says that humans avoid repeating experiences that end badly.

If you finish the fast after three or 3 1/2 days, that will be a “bad ending“ - not because you will have been unsuccessful, because you will have been miserable.

I tell everybody that will listen to me: if you do a short fast, do only one or at the most two days. If you’re going to go longer than that, make it seven days.

You will feel like a champion after seven days. After the first three days, it’s a lot easier anyway.