For me it always used to be after the fast day closest to Monday. That was when I was weighing once a week at the health clinic at work.
Now that I'm weighing daily, I have cheated a few times and used the weight the day after if it was lower than my normal weigh-in day. Two weeks running it has been lower, in both cases resulting in a small loss for the week, whereas the normal weigh in time would have resulted in a small gain for the week.
If my weight goes even lower later in the week, I haven't gone so far as putting it in the progress tracker, though I'm tempted... [I'm on a bit of a plateau, or at least very slowed down weight loss compared to the first 3 or so months I've been doing 4:3. Any little helps psychologically.]
p.s. I'm using True Weight to record my daily weigh-ins, btw. I'm a little disappointed in the trend line. I'm not sure what equation they use to get it, but it seems almost always to be higher than my weigh-in numbers, which just seems mean.