Most people want to weigh themselves when they are at their lowest, so it would make sense to weigh first thing on a Friday morning. Some however, jump on the scales every morning and become distressed due to the natural fluctuations in their weight between fasting and non fasting days, whereas others weigh every day and chart the general trend, understanding that weight can fluctuate by several pounds for various reasons. It is really up to you, but you need to rememebr that the average weight loss for women doing 5:2 is only 1lb per week. It is a slow loss and not always regular. You might lose 2lbs and then not lose anything for 2 or 3 weeks.
You’ll have to experiment and work out what suits you.